Mypy Release Notes

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Mypy 1.18

We’ve just uploaded mypy 1.18 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Mypy Performance Improvements

Mypy 1.18 includes numerous performance improvements, resulting in about 40% speedup compared to 1.17 when type checking mypy itself. In extreme cases, the improvement can be 10x or higher. The list below is an overview of the various mypy optimizations. Many mypyc improvements (discussed in a separate section below) also improve performance.

Type caching optimizations have a small risk of causing regressions. When reporting issues with unexpected inferred types, please also check if --disable-expression-cache will work around the issue, as it turns off some of these optimizations.

Improve self check performance by 1.8% (Jukka Lehtosalo, PR 19768, 19769, 19770)
Optimize fixed-format deserialization (Ivan Levkivskyi, PR 19765)
Use macros to optimize fixed-format deserialization (Ivan Levkivskyi, PR 19757)
Two additional micro‑optimizations (Ivan Levkivskyi, PR 19627)
Another set of micro‑optimizations (Ivan Levkivskyi, PR 19633)
Cache common types (Ivan Levkivskyi, PR 19621)
Skip more method bodies in third‑party libraries for speed (Ivan Levkivskyi, PR 19586)
Simplify the representation of callable types (Ivan Levkivskyi, PR 19580)
Add cache for types of some expressions (Ivan Levkivskyi, PR 19505)
Use cache for dictionary expressions (Ivan Levkivskyi, PR 19536)
Use cache for binary operations (Ivan Levkivskyi, PR 19523)
Cache types of type objects (Ivan Levkivskyi, PR 19514)
Avoid duplicate work when checking boolean operations (Ivan Levkivskyi, PR 19515)
Optimize generic inference passes (Ivan Levkivskyi, PR 19501)
Speed up the default plugin (Jukka Lehtosalo, PRs 19385 and 19462)
Remove nested imports from the default plugin (Ivan Levkivskyi, PR 19388)
Micro‑optimize type expansion (Jukka Lehtosalo, PR 19461)
Micro‑optimize type indirection (Jukka Lehtosalo, PR 19460)
Micro‑optimize the plugin framework (Jukka Lehtosalo, PR 19464)
Avoid temporary set creation in subtype checking (Jukka Lehtosalo, PR 19463)
Subtype checking micro‑optimization (Jukka Lehtosalo, PR 19384)
Return early where possible in subtype check (Stanislav Terliakov, PR 19400)
Deduplicate some types before joining (Stanislav Terliakov, PR 19409)
Speed up type checking by caching argument inference context (Jukka Lehtosalo, PR 19323)
Optimize binding method self argument type and deprecation checks (Ivan Levkivskyi, PR 19556)
Keep trivial instance types/aliases during expansion (Ivan Levkivskyi, PR 19543)

Fixed‑Format Cache (Experimental)

Mypy now supports a new cache format used for faster incremental builds. It makes incremental builds up to twice as fast. The feature is experimental and currently only supported when using a compiled version of mypy. Use --fixed-format-cache to enable the new format, or fixed_format_cache = True in a configuration file.

We plan to enable this by default in a future mypy release, and we'll eventually deprecate and remove support for the original JSON-based format.

Unlike the JSON-based cache format, the new binary format is currently not easy to parse and inspect by mypy users. We are planning to provide a tool to convert fixed-format cache files to JSON, but details of the output JSON may be different from the current JSON format. If you rely on being able to inspect mypy cache files, we recommend creating a GitHub issue and explaining your use case, so that we can more likely provide support for it. (Using MypyFile.read(binary_data) to inspect cache data may be sufficient to support some use cases.)

This feature was contributed by Ivan Levkivskyi (PR 19668, 19735, 19750, 19681, 19752, 19815).

Flexible Variable Definitions: Update

Mypy 1.16.0 introduced --allow-redefinition-new, which allows redefining variables with different types, and inferring union types for variables from multiple assignments. The feature is now documented in the --help output, but the feature is still experimental.

We are planning to enable this by default in mypy 2.0, and we will also deprecate the older --allow-redefinition flag. Since the new behavior differs significantly from the older flag, we encourage users of --allow-redefinition to experiment with --allow-redefinition-new and create a GitHub issue if the new functionality doesn't support some important use cases.

This feature was contributed by Jukka Lehtosalo.

Inferred Type for Bare ClassVar

A ClassVar without an explicit type annotation now causes the type of the variable to be inferred from the initializer:

from typing import ClassVar

class Item:
    # Type of 'next_id' is now 'int' (it was 'Any')
    next_id: ClassVar = 1

    ...

This feature was contributed by Ivan Levkivskyi (PR 19573).

Disjoint Base Classes (@disjoint_base, PEP 800)

Mypy now understands disjoint bases (PEP 800): it recognizes the @disjoint_base decorator, and rejects class definitions that combine mutually incompatible base classes, and takes advantage of the fact that such classes cannot exist in reachability and narrowing logic.

This class definition will now generate an error:

# Error: Class "Bad" has incompatible disjoint bases
class Bad(str, Exception):
    ...

This feature was contributed by Jelle Zijlstra (PR 19678).

Miscellaneous New Mypy Features

Add --strict-equality-for-none to flag non-overlapping comparisons involving None (Christoph Tyralla, PR 19718)
Don’t show import‑related errors after a module‑level assert such as assert sys.platform == "linux" that is always false (Stanislav Terliakov, PR 19347)

Improvements to Match Statements

Add temporary named expressions for match subjects (Stanislav Terliakov, PR 18446)
Fix unwrapping of assignment expressions in match subject (Marc Mueller, PR 19742)
Omit errors for class patterns against object (Marc Mueller, PR 19709)
Remove unnecessary error for certain match class patterns (Marc Mueller, PR 19708)
Use union type for captured vars in or pattern (Marc Mueller, PR 19710)
Prevent final reassignment inside match case (Omer Hadari, PR 19496)

Fixes to Crashes

Fix crash with variadic tuple arguments to a generic type (Randolf Scholz, PR 19705)
Fix crash when enable_error_code in pyproject.toml has wrong type (wyattscarpenter, PR 19494)
Prevent crash for dataclass with PEP 695 TypeVarTuple on Python 3.13+ (Stanislav Terliakov, PR 19565)
Fix crash on settable property alias (Ivan Levkivskyi, PR 19615)

Experimental Free-threading Support for Mypyc

All mypyc tests now pass on free-threading Python 3.14 release candidate builds. The performance of various micro-benchmarks scale well across multiple threads.

Free-threading support is still experimental. Note that native attribute access (get and set), list item access and certain other operations are still unsafe when there are race conditions. This will likely change in the future. You can follow the area-free-threading label in the mypyc issues tracker to follow progress.

Related PRs:

Enable free‑threading when compiling multiple modules (Jukka Lehtosalo, PR 19541)
Fix list.pop on free‑threaded builds (Jukka Lehtosalo, PR 19522)
Make type objects immortal under free‑threading (Jukka Lehtosalo, PR 19538)

Mypyc: Support `new`

Mypyc now has rudimentary support for user-defined __new__ methods.

This feature was contributed by Piotr Sawicki (PR 19739).

Mypyc: Faster Generators and Async Functions

Generators and calls of async functions are now faster, sometimes by 2x or more.

Related PRs:

Speed up for loops over native generators (Jukka Lehtosalo, PR 19415)
Speed up native‑to‑native calls using await (Jukka Lehtosalo, PR 19398)
Call generator helper directly in await expressions (Jukka Lehtosalo, PR 19376)
Speed up generator allocation with per‑type freelists (Jukka Lehtosalo, PR 19316)

Miscellaneous Mypyc Improvements

Special‑case certain Enum method calls for speed (Ivan Levkivskyi, PR 19634)
Fix issues related to subclassing and undefined attribute tracking (Chainfire, PR 19787)
Fix invalid C function signature (Jukka Lehtosalo, PR 19773)
Speed up implicit __ne__ (Jukka Lehtosalo, PR 19759)
Speed up equality with optional str/bytes types (Jukka Lehtosalo, PR 19758)
Speed up access to empty tuples (BobTheBuidler, PR 19654)
Speed up calls with *args (BobTheBuidler, PRs 19623 and 19631)
Speed up calls with **kwargs (BobTheBuidler, PR 19630)
Optimize type(x), x.__class__, and <type>.__name__ (Jukka Lehtosalo, PR 19691, 19683)
Specialize bytes.decode for common encodings (Jukka Lehtosalo, PR 19688)
Speed up in operations using final fixed‑length tuples (Jukka Lehtosalo, PR 19682)
Optimize f‑string building from final values (BobTheBuidler, PR 19611)
Add dictionary set item for exact dict instances (BobTheBuidler, PR 19657)
Cache length when iterating over immutable types (BobTheBuidler, PR 19656)
Fix name conflict related to attributes of generator classes (Piotr Sawicki, PR 19535)
Fix segfault from heap type objects with a static docstring (Brian Schubert, PR 19636)
Unwrap NewType to its base type for additional optimizations (BobTheBuidler, PR 19497)
Generate an export table only for separate compilation (Jukka Lehtosalo, PR 19521)
Speed up isinstance with built‑in types (Piotr Sawicki, PR 19435)
Use native integers for some sequence indexing (Jukka Lehtosalo, PR 19426)
Speed up isinstance(obj, list) (Piotr Sawicki, PR 19416)
Report error on reserved method names (Piotr Sawicki, PR 19407)
Speed up string equality (Jukka Lehtosalo, PR 19402)
Raise NameError on undefined names (Piotr Sawicki, PR 19395)
Use per‑type freelists for nested functions (Jukka Lehtosalo, PR 19390)
Simplify comparison of tuple elements (Piotr Sawicki, PR 19396)
Generate introspection signatures for compiled functions (Brian Schubert, PR 19307)
Fix undefined attribute checking special case (Jukka Lehtosalo, PR 19378)
Fix comparison of tuples with different lengths (Piotr Sawicki, PR 19372)
Speed up list.clear (Jahongir Qurbonov, PR 19344)
Speed up weakref.proxy (BobTheBuidler, PR 19217)
Speed up weakref.ref (BobTheBuidler, PR 19099)
Speed up str.count (BobTheBuidler, PR 19264)

Stubtest Improvements

Add temporary --ignore-disjoint-bases flag to ease PEP 800 migration (Joren Hammudoglu, PR 19740)
Flag redundant uses of @disjoint_base (Jelle Zijlstra, PR 19715)
Improve signatures for __init__ of C extension classes (Stephen Morton, PR 18259)
Handle overloads with mixed positional‑only parameters (Stephen Morton, PR 18287)
Use “parameter” (not “argument”) in error messages (PrinceNaroliya, PR 19707)
Don’t require @disjoint_base when __slots__ imply finality (Jelle Zijlstra, PR 19701)
Allow runtime‑existing aliases of @type_check_only types (Brian Schubert, PR 19568)
More detailed checking of type objects in stubtest (Stephen Morton, PR 18251)
Support running stubtest in non-UTF8 terminals (Stanislav Terliakov, PR 19085)

Documentation Updates

Add idlemypyextension to IDE integrations (CoolCat467, PR 18615)
Document that object is often preferable to Any in APIs (wyattscarpenter, PR 19103)
Include a detailed listing of flags enabled by --strict (wyattscarpenter, PR 19062)
Update “common issues” (reveal_type/reveal_locals; note on orjson) (wyattscarpenter, PR 19059, 19058)

Other Notable Fixes and Improvements

Remove deprecated --new-type-inference flag (the new algorithm has long been default) (Ivan Levkivskyi, PR 19570)
Use empty context as a fallback for return expressions when outer context misleads inference (Ivan Levkivskyi, PR 19767)
Fix forward references in type parameters of over‑parameterized PEP 695 aliases (Brian Schubert, PR 19725)
Don’t expand PEP 695 aliases when checking node fullnames (Brian Schubert, PR 19699)
Don’t use outer context for ‘or’ expression inference when LHS is Any (Stanislav Terliakov, PR 19748)
Recognize buffer protocol special methods (Brian Schubert, PR 19581)
Support attribute access on enum members correctly (Stanislav Terliakov, PR 19422)
Check __slots__ assignments on self types (Stanislav Terliakov, PR 19332)
Move self‑argument checks after decorator application (Stanislav Terliakov, PR 19490)
Infer empty list for __slots__ and module __all__ (Stanislav Terliakov, PR 19348)
Use normalized tuples for fallback calculation (Stanislav Terliakov, PR 19111)
Preserve literals when joining similar types (Stanislav Terliakov, PR 19279)
Allow adjacent conditionally‑defined overloads (Stanislav Terliakov, PR 19042)
Check property decorators more strictly (Stanislav Terliakov, PR 19313)
Support properties with generic setters (Ivan Levkivskyi, PR 19298)
Generalize class/static method and property alias support (Ivan Levkivskyi, PR 19297)
Re‑widen custom properties after narrowing (Ivan Levkivskyi, PR 19296)
Avoid erasing type objects when checking runtime cover (Shantanu, PR 19320)
Include tuple fallback in constraints built from tuple types (Stanislav Terliakov, PR 19100)
Somewhat better isinstance support on old‑style unions (Shantanu, PR 19714)
Improve promotions inside unions (Christoph Tyralla, PR 19245)
Treat uninhabited types as having all attributes (Ivan Levkivskyi, PR 19300)
Improve metaclass conflict checks (Robsdedude, PR 17682)
Fixes to metaclass resolution algorithm (Robsdedude, PR 17713)
PEP 702 @deprecated: handle “combined” overloads (Christoph Tyralla, PR 19626)
PEP 702 @deprecated: include overloads in snapshot descriptions (Christoph Tyralla, PR 19613)
Ignore overload implementation when checking __OP__ / __rOP__ compatibility (Stanislav Terliakov, PR 18502)
Support _value_ as a fallback for ellipsis Enum members (Stanislav Terliakov, PR 19352)
Sort arguments in TypedDict overlap messages (Marc Mueller, PR 19666)
Fix handling of implicit return in lambda (Stanislav Terliakov, PR 19642)
Improve behavior of uninhabited types (Stanislav Terliakov, PR 19648)
Fix overload diagnostics when *args and **kwargs both match (Shantanu, PR 19614)
Further fix overload diagnostics for *args/**kwargs (Shantanu, PR 19619)
Show type variable name in “Cannot infer type argument” (Brian Schubert, PR 19290)
Fail gracefully on unsupported template strings (PEP 750) (Brian Schubert, PR 19700)
Revert colored argparse help for Python 3.14 (Marc Mueller, PR 19721)
Update stubinfo for latest typeshed (Shantanu, PR 19771)
Fix dict assignment when an incompatible same‑shape TypedDict exists (Stanislav Terliakov, PR 19592)
Fix constructor type for subclasses of Any (Ivan Levkivskyi, PR 19295)
Fix TypeGuard/TypeIs being forgotten in some cases (Brian Schubert, PR 19325)
Fix TypeIs negative narrowing for unions of generics (Brian Schubert, PR 18193)
dmypy suggest: Fix incorrect signature suggestion when a type matches a module name (Brian Schubert, PR 18937)
dmypy suggest: Fix interaction with __new__ (Stanislav Terliakov, PR 18966)
dmypy suggest: Support Callable / callable Protocols in decorator unwrapping (Anthony Sottile, PR 19072)
Fix missing error when redeclaring a type variable in a nested generic class (Brian Schubert, PR 18883)
Fix for overloaded type object erasure (Shantanu, PR 19338)
Fix TypeGuard with call on temporary object (Saul Shanabrook, PR 19577)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Ali Hamdan
Anthony Sottile
BobTheBuidler
Brian Schubert
Chainfire
Charlie Denton
Christoph Tyralla
CoolCat467
Daniel Hnyk
Emily
Emma Smith
Ethan Sarp
Ivan Levkivskyi
Jahongir Qurbonov
Jelle Zijlstra
Joren Hammudoglu
Jukka Lehtosalo
Marc Mueller
Omer Hadari
Piotr Sawicki
PrinceNaroliya
Randolf Scholz
Robsdedude
Saul Shanabrook
Shantanu
Stanislav Terliakov
Stephen Morton
wyattscarpenter

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.17

We’ve just uploaded mypy 1.17 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Optionally Check That Match Is Exhaustive

Mypy can now optionally generate an error if a match statement does not match exhaustively, without having to use assert_never(...). Enable this by using --enable-error-code exhaustive-match.

Example:

# mypy: enable-error-code=exhaustive-match

import enum

class Color(enum.Enum):
    RED = 1
    BLUE = 2

def show_color(val: Color) -> None:
    # error: Unhandled case for values of type "Literal[Color.BLUE]"
    match val:
        case Color.RED:
            print("red")

This feature was contributed by Donal Burns (PR 19144).

Further Improvements to Attribute Resolution

This release includes additional improvements to how attribute types and kinds are resolved. These fix many bugs and overall improve consistency.

Handle corner case: protocol/class variable/descriptor (Ivan Levkivskyi, PR 19277)
Fix a few inconsistencies in protocol/type object interactions (Ivan Levkivskyi, PR 19267)
Refactor/unify access to static attributes (Ivan Levkivskyi, PR 19254)
Remove inconsistencies in operator handling (Ivan Levkivskyi, PR 19250)
Make protocol subtyping more consistent (Ivan Levkivskyi, PR 18943)

Fixes to Nondeterministic Type Checking

Previous mypy versions could infer different types for certain expressions across different runs (typically depending on which order certain types were processed, and this order was nondeterministic). This release includes fixes to several such issues.

Fix nondeterministic type checking by making join with explicit Protocol and type promotion commute (Shantanu, PR 18402)
Fix nondeterministic type checking caused by nonassociative of None joins (Shantanu, PR 19158)
Fix nondeterministic type checking caused by nonassociativity of joins (Shantanu, PR 19147)
Fix nondeterministic type checking by making join between type and TypeVar commute (Shantanu, PR 19149)

Remove Support for Targeting Python 3.8

Mypy now requires --python-version 3.9 or greater. Support for targeting Python 3.8 is fully removed now. Since 3.8 is an unsupported version, mypy will default to the oldest supported version (currently 3.9) if you still try to target 3.8.

This change is necessary because typeshed stopped supporting Python 3.8 after it reached its End of Life in October 2024.

Contributed by Marc Mueller (PR 19157, PR 19162).

Initial Support for Python 3.14

Mypy is now tested on 3.14 and mypyc works with 3.14.0b3 and later. Binary wheels compiled with mypyc for mypy itself will be available for 3.14 some time after 3.14.0rc1 has been released.

Note that not all features are supported just yet.

Contributed by Marc Mueller (PR 19164)

Deprecated Flag: `--force-uppercase-builtins`

Mypy only supports Python 3.9+. The --force-uppercase-builtins flag is now deprecated as unnecessary, and a no-op. It will be removed in a future version.

Contributed by Marc Mueller (PR 19176)

Mypyc: Improvements to Generators and Async Functions

This release includes both performance improvements and bug fixes related to generators and async functions (these share many implementation details).

Fix exception swallowing in async try/finally blocks with await (Chainfire, PR 19353)
Fix AttributeError in async try/finally with mixed return paths (Chainfire, PR 19361)
Make generated generator helper method internal (Jukka Lehtosalo, PR 19268)
Free coroutine after await encounters StopIteration (Jukka Lehtosalo, PR 19231)
Use non-tagged integer for generator label (Jukka Lehtosalo, PR 19218)
Merge generator and environment classes in simple cases (Jukka Lehtosalo, PR 19207)

Mypyc: Partial, Unsafe Support for Free Threading

Mypyc has minimal, quite memory-unsafe support for the free threaded builds of 3.14. It is also only lightly tested. Bug reports and experience reports are welcome!

Here are some of the major limitations:

Free threading only works when compiling a single module at a time.
If there is concurrent access to an object while another thread is mutating the same object, it's possible to encounter segfaults and memory corruption.
There are no efficient native primitives for thread synthronization, though the regular threading module can be used.
Some workloads don't scale well to multiple threads for no clear reason.

Related PRs:

Enable partial, unsafe support for free-threading (Jukka Lehtosalo, PR 19167)
Fix incref/decref on free-threaded builds (Jukka Lehtosalo, PR 19127)

Other Mypyc Fixes and Improvements

Derive .c file name from full module name if using multi_file (Jukka Lehtosalo, PR 19278)
Support overriding the group name used in output files (Jukka Lehtosalo, PR 19272)
Add note about using non-native class to subclass built-in types (Jukka Lehtosalo, PR 19236)
Make some generated classes implicitly final (Jukka Lehtosalo, PR 19235)
Don't simplify module prefixes if using separate compilation (Jukka Lehtosalo, PR 19206)

Stubgen Improvements

Add import for types in __exit__ method signature (Alexey Makridenko, PR 19120)
Add support for including class and property docstrings (Chad Dombrova, PR 17964)
Don't generate Incomplete | None = None argument annotation (Sebastian Rittau, PR 19097)
Support several more constructs in stubgen's alias printer (Stanislav Terliakov, PR 18888)

Miscellaneous Fixes and Improvements

Combine the revealed types of multiple iteration steps in a more robust manner (Christoph Tyralla, PR 19324)
Improve the handling of “iteration dependent” errors and notes in finally clauses (Christoph Tyralla, PR 19270)
Lessen dmypy suggest path limitations for Windows machines (CoolCat467, PR 19337)
Fix type ignore comments erroneously marked as unused by dmypy (Charlie Denton, PR 15043)
Fix misspelled exhaustive-match error code (johnthagen, PR 19276)
Fix missing error context for unpacking assignment involving star expression (Brian Schubert, PR 19258)
Fix and simplify error de-duplication (Ivan Levkivskyi, PR 19247)
Disallow ClassVar in type aliases (Brian Schubert, PR 19263)
Add script that prints list of compiled files when compiling mypy (Jukka Lehtosalo, PR 19260)
Fix help message url for “None and Optional handling” section (Guy Wilson, PR 19252)
Display fully qualified name of imported base classes in errors about incompatible overrides (Mikhail Golubev, PR 19115)
Avoid false unreachable, redundant-expr, and redundant-casts warnings in loops more robustly and efficiently, and avoid multiple revealed type notes for the same line (Christoph Tyralla, PR 19118)
Fix type extraction from isinstance checks (Stanislav Terliakov, PR 19223)
Erase stray type variables in functools.partial (Stanislav Terliakov, PR 18954)
Make inferring condition value recognize the whole truth table (Stanislav Terliakov, PR 18944)
Support type aliases, NamedTuple and TypedDict in constrained TypeVar defaults (Stanislav Terliakov, PR 18884)
Move dataclass kw_only fields to the end of the signature (Stanislav Terliakov, PR 19018)
Provide a better fallback value for the python_version option (Marc Mueller, PR 19162)
Avoid spurious non-overlapping equality error with metaclass with __eq__ (Michael J. Sullivan, PR 19220)
Narrow type variable bounds (Ivan Levkivskyi, PR 19183)
Add classifier for Python 3.14 (Marc Mueller, PR 19199)
Capitalize syntax error messages (Charulata, PR 19114)
Infer constraints eagerly if actual is Any (Ivan Levkivskyi, PR 19190)
Include walrus assignments in conditional inference (Stanislav Terliakov, PR 19038)
Use PEP 604 syntax when converting types to strings (Marc Mueller, PR 19179)
Use more lower-case builtin types in error messages (Marc Mueller, PR 19177)
Fix example to use correct method of Stack (Łukasz Kwieciński, PR 19123)
Forbid .pop of Readonly NotRequired TypedDict items (Stanislav Terliakov, PR 19133)
Emit a friendlier warning on invalid exclude regex, instead of a stacktrace (wyattscarpenter, PR 19102)
Enable ANSI color codes for dmypy client in Windows (wyattscarpenter, PR 19088)
Extend special case for context-based type variable inference to unions in return position (Stanislav Terliakov, PR 18976)

Mypy 1.17.1

Retain None as constraints bottom if no bottoms were provided (Stanislav Terliakov, PR 19485)
Fix “ignored exception in hasattr” in dmypy (Stanislav Terliakov, PR 19428)
Prevent a crash when InitVar is redefined with a method in a subclass (Stanislav Terliakov, PR 19453)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alexey Makridenko
Brian Schubert
Chad Dombrova
Chainfire
Charlie Denton
Charulata
Christoph Tyralla
CoolCat467
Donal Burns
Guy Wilson
Ivan Levkivskyi
johnthagen
Jukka Lehtosalo
Łukasz Kwieciński
Marc Mueller
Michael J. Sullivan
Mikhail Golubev
Sebastian Rittau
Shantanu
Stanislav Terliakov
wyattscarpenter

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.16

We’ve just uploaded mypy 1.16 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Different Property Getter and Setter Types

Mypy now supports using different types for a property getter and setter:

class A:
    _value: int

    @property
    def foo(self) -> int:
        return self._value

    @foo.setter
    def foo(self, x: str | int) -> None:
        try:
            self._value = int(x)
        except ValueError:
            raise Exception(f"'{x}' is not a valid value for 'foo'")

This was contributed by Ivan Levkivskyi (PR 18510).

Flexible Variable Redefinitions (Experimental)

Mypy now allows unannotated variables to be freely redefined with different types when using the experimental --allow-redefinition-new flag. You will also need to enable --local-partial-types. Mypy will now infer a union type when different types are assigned to a variable:

# mypy: allow-redefinition-new, local-partial-types

def f(n: int, b: bool) -> int | str:
    if b:
        x = n
    else:
        x = str(n)
    # Type of 'x' is int | str here.
    return x

Without the new flag, mypy only supports inferring optional types (X | None) from multiple assignments, but now mypy can infer arbitrary union types.

An unannotated variable can now also have different types in different code locations:

# mypy: allow-redefinition-new, local-partial-types
...

if cond():
    for x in range(n):
        # Type of 'x' is 'int' here
        ...
else:
    for x in ['a', 'b']:
        # Type of 'x' is 'str' here
        ...

We are planning to turn this flag on by default in mypy 2.0, along with --local-partial-types. The feature is still experimental and has known issues, and the semantics may still change in the future. You may need to update or add type annotations when switching to the new behavior, but if you encounter anything unexpected, please create a GitHub issue.

This was contributed by Jukka Lehtosalo (PR 18727, PR 19153).

Stricter Type Checking with Imprecise Types

Mypy can now detect additional errors in code that uses Any types or has missing function annotations.

When calling dict.get(x, None) on an object of type dict[str, Any], this now results in an optional type (in the past it was Any):

def f(d: dict[str, Any]) -> int:
    # Error: Return value has type "Any | None" but expected "int"
    return d.get("x", None)

Type narrowing using assignments can result in more precise types in the presence of Any types:

def foo(): ...

def bar(n: int) -> None:
    x = foo()
    # Type of 'x' is 'Any' here
    if n > 5:
        x = str(n)
        # Type of 'x' is 'str' here

When using --check-untyped-defs, unannotated overrides are now checked more strictly against superclass definitions.

Related PRs:

Use union types instead of join in binder (Ivan Levkivskyi, PR 18538)
Check superclass compatibility of untyped methods if --check-untyped-defs is set (Stanislav Terliakov, PR 18970)

Improvements to Attribute Resolution

This release includes several fixes to inconsistent resolution of attribute, method and descriptor types.

Consolidate descriptor handling (Ivan Levkivskyi, PR 18831)
Make multiple inheritance checking use common semantics (Ivan Levkivskyi, PR 18876)
Make method override checking use common semantics (Ivan Levkivskyi, PR 18870)
Fix descriptor overload selection (Ivan Levkivskyi, PR 18868)
Handle union types when binding self (Ivan Levkivskyi, PR 18867)
Make variable override checking use common semantics (Ivan Levkivskyi, PR 18847)
Make descriptor handling behave consistently (Ivan Levkivskyi, PR 18831)

Make Implementation for Abstract Overloads Optional

The implementation can now be omitted for abstract overloaded methods, even outside stubs:

from abc import abstractmethod
from typing import overload

class C:
    @abstractmethod
    @overload
    def foo(self, x: int) -> int: ...

    @abstractmethod
    @overload
    def foo(self, x: str) -> str: ...

    # No implementation required for "foo"

This was contributed by Ivan Levkivskyi (PR 18882).

Option to Exclude Everything in .gitignore

You can now use --exclude-gitignore to exclude everything in a .gitignore file from the mypy build. This behaves similar to excluding the paths using --exclude. We might enable this by default in a future mypy release.

This was contributed by Ivan Levkivskyi (PR 18696).

Selectively Disable Deprecated Warnings

It's now possible to selectively disable warnings generated from warnings.deprecated using the --deprecated-calls-exclude option:

# mypy --enable-error-code deprecated
#      --deprecated-calls-exclude=foo.A
import foo

foo.A().func()  # OK, the deprecated warning is ignored

# file foo.py

from typing_extensions import deprecated

class A:
    @deprecated("Use A.func2 instead")
    def func(self): pass

    ...

Contributed by Marc Mueller (PR 18641)

Annotating Native/Non-Native Classes in Mypyc

You can now declare a class as a non-native class when compiling with mypyc. Unlike native classes, which are extension classes and have an immutable structure, non-native classes are normal Python classes at runtime and are fully dynamic. Example:

from mypy_extensions import mypyc_attr

@mypyc_attr(native_class=False)
class NonNativeClass:
    ...

o = NonNativeClass()

# Ok, even if attribute "foo" not declared in class body
setattr(o, "foo", 1)

Classes are native by default in compiled modules, but classes that use certain features (such as most metaclasses) are implicitly non-native.

You can also explicitly declare a class as native. In this case mypyc will generate an error if it can't compile the class as a native class, instead of falling back to a non-native class:

from mypy_extensions import mypyc_attr
from foo import MyMeta

# Error: Unsupported metaclass for a native class
@mypyc_attr(native_class=True)
class C(metaclass=MyMeta):
    ...

Since native classes are significantly more efficient that non-native classes, you may want to ensure that certain classes always compiled as native classes.

This feature was contributed by Valentin Stanciu (PR 18802).

Mypyc Fixes and Improvements

Improve documentation of native and non-native classes (Jukka Lehtosalo, PR 19154)
Fix compilation when using Python 3.13 debug build (Valentin Stanciu, PR 19045)
Show the reason why a class can't be a native class (Valentin Stanciu, PR 19016)
Support await/yield while temporary values are live (Michael J. Sullivan, PR 16305)
Fix spilling values with overlapping error values (Jukka Lehtosalo, PR 18961)
Fix reference count of spilled register in async def (Jukka Lehtosalo, PR 18957)
Add basic optimization for sorted (Marc Mueller, PR 18902)
Fix access of class object in a type annotation (Advait Dixit, PR 18874)
Optimize list.__imul__ and tuple.__mul__ (Marc Mueller, PR 18887)
Optimize list.__add__, list.__iadd__ and tuple.__add__ (Marc Mueller, PR 18845)
Add and implement primitive list.copy() (exertustfm, PR 18771)
Optimize builtins.repr (Marc Mueller, PR 18844)
Support iterating over keys/values/items of dict-bound TypeVar and ParamSpec.kwargs (Stanislav Terliakov, PR 18789)
Add efficient primitives for str.strip() etc. (Advait Dixit, PR 18742)
Document that strip() etc. are optimized (Jukka Lehtosalo, PR 18793)
Fix mypyc crash with enum type aliases (Valentin Stanciu, PR 18725)
Optimize str.find and str.rfind (Marc Mueller, PR 18709)
Optimize str.__contains__ (Marc Mueller, PR 18705)
Fix order of steal/unborrow in tuple unpacking (Ivan Levkivskyi, PR 18732)
Optimize str.partition and str.rpartition (Marc Mueller, PR 18702)
Optimize str.startswith and str.endswith with tuple argument (Marc Mueller, PR 18678)
Improve str.startswith and str.endswith with tuple argument (Marc Mueller, PR 18703)
pythoncapi_compat: don't define Py_NULL if it is already defined (Michael R. Crusoe, PR 18699)
Optimize str.splitlines (Marc Mueller, PR 18677)
Mark dict.setdefault as optimized (Marc Mueller, PR 18685)
Support __del__ methods (Advait Dixit, PR 18519)
Optimize str.rsplit (Marc Mueller, PR 18673)
Optimize str.removeprefix and str.removesuffix (Marc Mueller, PR 18672)
Recognize literal types in __match_args__ (Stanislav Terliakov, PR 18636)
Fix non extension classes with attribute annotations using forward references (Valentin Stanciu, PR 18577)
Use lower-case generic types such as list[t] in documentation (Jukka Lehtosalo, PR 18576)
Improve support for frozenset (Marc Mueller, PR 18571)
Fix wheel build for cp313-win (Marc Mueller, PR 18560)
Reduce impact of immortality (introduced in Python 3.12) on reference counting performance (Jukka Lehtosalo, PR 18459)
Update math error messages for 3.14 (Marc Mueller, PR 18534)
Update math error messages for 3.14 (2) (Marc Mueller, PR 18949)
Replace deprecated _PyLong_new with PyLongWriter API (Marc Mueller, PR 18532)

Fixes to Crashes

Traverse module ancestors when traversing reachable graph nodes during dmypy update (Stanislav Terliakov, PR 18906)
Fix crash on multiple unpacks in a bare type application (Stanislav Terliakov, PR 18857)
Prevent crash when enum/TypedDict call is stored as a class attribute (Stanislav Terliakov, PR 18861)
Fix crash on multiple unpacks in a bare type application (Stanislav Terliakov, PR 18857)
Fix crash on type inference against non-normal callables (Ivan Levkivskyi, PR 18858)
Fix crash on decorated getter in settable property (Ivan Levkivskyi, PR 18787)
Fix crash on callable with *args and suffix against Any (Ivan Levkivskyi, PR 18781)
Fix crash on deferred supertype and setter override (Ivan Levkivskyi, PR 18649)
Fix crashes on incorrectly detected recursive aliases (Ivan Levkivskyi, PR 18625)
Report that NamedTuple and dataclass are incompatile instead of crashing (Christoph Tyralla, PR 18633)
Fix mypy daemon crash (Valentin Stanciu, PR 19087)

Performance Improvements

These are specific to mypy. Mypyc-related performance improvements are discussed elsewhere.

Speed up binding self in trivial cases (Ivan Levkivskyi, PR 19024)
Small constraint solver optimization (Aaron Gokaslan, PR 18688)

Documentation Updates

Improve documentation of --strict (lenayoung8, PR 18903)
Remove a note about from __future__ import annotations (Ageev Maxim, PR 18915)
Improve documentation on type narrowing (Tim Hoffmann, PR 18767)
Fix metaclass usage example (Georg, PR 18686)
Update documentation on extra_checks flag (Ivan Levkivskyi, PR 18537)

Stubgen Improvements

Fix TypeAlias handling (Alexey Makridenko, PR 18960)
Handle arg=None in C extension modules (Anthony Sottile, PR 18768)
Fix valid type detection to allow pipe unions (Chad Dombrova, PR 18726)
Include simple decorators in stub files (Marc Mueller, PR 18489)
Support positional and keyword-only arguments in stubdoc (Paul Ganssle, PR 18762)
Fall back to Incomplete if we are unable to determine the module name (Stanislav Terliakov, PR 19084)

Stubtest Improvements

Make stubtest ignore __slotnames__ (Nick Pope, PR 19077)
Fix stubtest tests on 3.14 (Jelle Zijlstra, PR 19074)
Support for strict_bytes in stubtest (Joren Hammudoglu, PR 19002)
Understand override (Shantanu, PR 18815)
Better checking of runtime arguments with dunder names (Shantanu, PR 18756)
Ignore setattr and delattr inherited from object (Stephen Morton, PR 18325)

Miscellaneous Fixes and Improvements

Add --strict-bytes to --strict (wyattscarpenter, PR 19049)
Admit that Final variables are never redefined (Stanislav Terliakov, PR 19083)
Add special support for @django.cached_property needed in django-stubs (sobolevn, PR 18959)
Do not narrow types to Never with binder (Ivan Levkivskyi, PR 18972)
Local forward references should precede global forward references (Ivan Levkivskyi, PR 19000)
Do not cache module lookup results in incremental mode that may become invalid (Stanislav Terliakov, PR 19044)
Only consider meta variables in ambiguous “any of” constraints (Stanislav Terliakov, PR 18986)
Allow accessing __init__ on final classes and when __init__ is final (Stanislav Terliakov, PR 19035)
Treat varargs as positional-only (A5rocks, PR 19022)
Enable colored output for argparse help in Python 3.14 (Marc Mueller, PR 19021)
Fix argparse for Python 3.14 (Marc Mueller, PR 19020)
dmypy suggest can now suggest through contextmanager-based decorators (Anthony Sottile, PR 18948)
Fix __r<magic_methods>__ being used under the same __<magic_method>__ hook (Arnav Jain, PR 18995)
Prioritize .pyi from -stubs packages over bundled .pyi (Joren Hammudoglu, PR 19001)
Fix missing subtype check case for type[T] (Stanislav Terliakov, PR 18975)
Fixes to the detection of redundant casts (Anthony Sottile, PR 18588)
Make some parse errors non-blocking (Shantanu, PR 18941)
Fix PEP 695 type alias with a mix of type arguments (PEP 696) (Marc Mueller, PR 18919)
Allow deeper recursion in mypy daemon, better error reporting (Carter Dodd, PR 17707)
Fix swapped errors for frozen/non-frozen dataclass inheritance (Nazrawi Demeke, PR 18918)
Fix incremental issue with namespace packages (Shantanu, PR 18907)
Exclude irrelevant members when narrowing union overlapping with enum (Stanislav Terliakov, PR 18897)
Flatten union before contracting literals when checking subtyping (Stanislav Terliakov, PR 18898)
Do not add kw_only dataclass fields to __match_args__ (sobolevn, PR 18892)
Fix error message when returning long tuple with type mismatch (Thomas Mattone, PR 18881)
Treat TypedDict (old-style) aliases as regular TypedDicts (Stanislav Terliakov, PR 18852)
Warn about unused type: ignore comments when error code is disabled (Brian Schubert, PR 18849)
Reject duplicate ParamSpec.{args,kwargs} at call site (Stanislav Terliakov, PR 18854)
Make detection of enum members more consistent (sobolevn, PR 18675)
Admit that **kwargs mapping subtypes may have no direct type parameters (Stanislav Terliakov, PR 18850)
Don't suggest types-setuptools for pkg_resources (Shantanu, PR 18840)
Suggest scipy-stubs for scipy as non-typeshed stub package (Joren Hammudoglu, PR 18832)
Narrow tagged unions in match statements (Gene Parmesan Thomas, PR 18791)
Consistently store settable property type (Ivan Levkivskyi, PR 18774)
Do not blindly undefer on leaving function (Ivan Levkivskyi, PR 18674)
Process superclass methods before subclass methods in semanal (Ivan Levkivskyi, PR 18723)
Only defer top-level functions (Ivan Levkivskyi, PR 18718)
Add one more type-checking pass (Ivan Levkivskyi, PR 18717)
Properly account for member and nonmember in enums (sobolevn, PR 18559)
Fix instance vs tuple subtyping edge case (Ivan Levkivskyi, PR 18664)
Improve handling of Any/object in variadic generics (Ivan Levkivskyi, PR 18643)
Fix handling of named tuples in class match pattern (Ivan Levkivskyi, PR 18663)
Fix regression for user config files (Shantanu, PR 18656)
Fix dmypy socket issue on GNU/Hurd (Mattias Ellert, PR 18630)
Don't assume that for loop body index variable is always set (Jukka Lehtosalo, PR 18631)
Fix overlap check for variadic generics (Ivan Levkivskyi, PR 18638)
Improve support for functools.partial of overloaded callable protocol (Shantanu, PR 18639)
Allow lambdas in except* clauses (Stanislav Terliakov, PR 18620)
Fix trailing commas in many multiline string options in pyproject.toml (sobolevn, PR 18624)
Allow trailing commas for files setting in mypy.ini and setup.ini (sobolevn, PR 18621)
Fix “not callable” issue for @dataclass(frozen=True) with Final attr (sobolevn, PR 18572)
Add missing TypedDict special case when checking member access (Stanislav Terliakov, PR 18604)
Use lower case list and dict in invariance notes (Jukka Lehtosalo, PR 18594)
Fix inference when class and instance match protocol (Ivan Levkivskyi, PR 18587)
Remove support for builtins.Any (Marc Mueller, PR 18578)
Update the overlapping check for tuples to account for NamedTuples (A5rocks, PR 18564)
Fix @deprecated (PEP 702) with normal overloaded methods (Christoph Tyralla, PR 18477)
Start propagating end columns/lines for type-arg errors (A5rocks, PR 18533)
Improve handling of type(x) is Foo checks (Stanislav Terliakov, PR 18486)
Suggest typing.Literal for exit-return error messages (Marc Mueller, PR 18541)
Allow redefinitions in except/else/finally (Stanislav Terliakov, PR 18515)
Disallow setting Python version using inline config (Shantanu, PR 18497)
Improve type inference in tuple multiplication plugin (Shantanu, PR 18521)
Add missing line number to yield from with wrong type (Stanislav Terliakov, PR 18518)
Hint at argument names when formatting callables with compatible return types in error messages (Stanislav Terliakov, PR 18495)
Add better naming and improve compatibility for ad hoc intersections of instances (Christoph Tyralla, PR 18506)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

A5rocks
Aaron Gokaslan
Advait Dixit
Ageev Maxim
Alexey Makridenko
Ali Hamdan
Anthony Sottile
Arnav Jain
Brian Schubert
bzoracler
Carter Dodd
Chad Dombrova
Christoph Tyralla
Dimitri Papadopoulos Orfanos
Emma Smith
exertustfm
Gene Parmesan Thomas
Georg
Ivan Levkivskyi
Jared Hance
Jelle Zijlstra
Joren Hammudoglu
lenayoung8
Marc Mueller
Mattias Ellert
Michael J. Sullivan
Michael R. Crusoe
Nazrawi Demeke
Nick Pope
Paul Ganssle
Shantanu
sobolevn
Stanislav Terliakov
Stephen Morton
Thomas Mattone
Tim Hoffmann
Tim Ruffing
Valentin Stanciu
Wesley Collin Wright
wyattscarpenter

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.15

We’ve just uploaded mypy 1.15 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Performance Improvements

Mypy is up to 40% faster in some use cases. This improvement comes largely from tuning the performance of the garbage collector. Additionally, the release includes several micro-optimizations that may be impactful for large projects.

Contributed by Jukka Lehtosalo

PR 18306
PR 18302
PR 18298
PR 18299

Mypyc Accelerated Mypy Wheels for ARM Linux

For best performance, mypy can be compiled to C extension modules using mypyc. This makes mypy 3-5x faster than when interpreted with pure Python. We now build and upload mypyc accelerated mypy wheels for manylinux_aarch64 to PyPI, making it easy for Linux users on ARM platforms to realise this speedup -- just pip install the latest mypy.

Contributed by Christian Bundy and Marc Mueller (PR mypy_mypyc-wheels#76, PR mypy_mypyc-wheels#89).

`--strict-bytes`

By default, mypy treats bytearray and memoryview values as assignable to the bytes type, for historical reasons. Use the --strict-bytes flag to disable this behavior. PEP 688 specified the removal of this special case. The flag will be enabled by default in mypy 2.0.

Contributed by Ali Hamdan (PR 18263) and Shantanu Jain (PR 13952).

Improvements to Reachability Analysis and Partial Type Handling in Loops

This change results in mypy better modelling control flow within loops and hence detecting several previously ignored issues. In some cases, this change may require additional explicit variable annotations.

Contributed by Christoph Tyralla (PR 18180, PR 18433).

(Speaking of partial types, remember that we plan to enable --local-partial-types by default in mypy 2.0.)

Better Discovery of Configuration Files

Mypy will now walk up the filesystem (up until a repository or file system root) to discover configuration files. See the mypy configuration file documentation for more details.

Contributed by Mikhail Shiryaev and Shantanu Jain (PR 16965, PR 18482)

Better Line Numbers for Decorators and Slice Expressions

Mypy now uses more correct line numbers for decorators and slice expressions. In some cases, you may have to change the location of a # type: ignore comment.

Contributed by Shantanu Jain (PR 18392, PR 18397).

Drop Support for Python 3.8

Mypy no longer supports running with Python 3.8, which has reached end-of-life. When running mypy with Python 3.9+, it is still possible to type check code that needs to support Python 3.8 with the --python-version 3.8 argument. Support for this will be dropped in the first half of 2025!

Contributed by Marc Mueller (PR 17492).

Mypyc Improvements

Fix __init__ for classes with @attr.s(slots=True) (Advait Dixit, PR 18447)
Report error for nested class instead of crashing (Valentin Stanciu, PR 18460)
Fix InitVar for dataclasses (Advait Dixit, PR 18319)
Remove unnecessary mypyc files from wheels (Marc Mueller, PR 18416)
Fix issues with relative imports (Advait Dixit, PR 18286)
Add faster primitive for some list get item operations (Jukka Lehtosalo, PR 18136)
Fix iteration over NamedTuple objects (Advait Dixit, PR 18254)
Mark mypyc package with py.typed (bzoracler, PR 18253)
Fix list index while checking for Enum class (Advait Dixit, PR 18426)

Stubgen Improvements

Improve dataclass init signatures (Marc Mueller, PR 18430)
Preserve dataclass_transform decorator (Marc Mueller, PR 18418)
Fix UnpackType for 3.11+ (Marc Mueller, PR 18421)
Improve self annotations (Marc Mueller, PR 18420)
Print InspectError traceback in stubgen walk_packages when verbose is specified (Gareth, PR 18224)

Stubtest Improvements

Fix crash with numpy array default values (Ali Hamdan, PR 18353)
Distinguish metaclass attributes from class attributes (Stephen Morton, PR 18314)

Fixes to Crashes

Prevent crash with Unpack of a fixed tuple in PEP695 type alias (Stanislav Terliakov, PR 18451)
Fix crash with --cache-fine-grained --cache-dir=/dev/null (Shantanu, PR 18457)
Prevent crashing when match arms use name of existing callable (Stanislav Terliakov, PR 18449)
Gracefully handle encoding errors when writing to stdout (Brian Schubert, PR 18292)
Prevent crash on generic NamedTuple with unresolved typevar bound (Stanislav Terliakov, PR 18585)

Documentation Updates

Add inline tabs to documentation (Marc Mueller, PR 18262)
Document any TYPE_CHECKING name works (Shantanu, PR 18443)
Update documentation to not mention 3.8 where possible (sobolevn, PR 18455)
Mention ignore_errors in exclude documentation (Shantanu, PR 18412)
Add Self misuse to common issues (Shantanu, PR 18261)

Other Notable Fixes and Improvements

Fix literal context for ternary expressions (Ivan Levkivskyi, PR 18545)
Ignore dataclass.__replace__ LSP violations (Marc Mueller, PR 18464)
Bind self to the class being defined when checking multiple inheritance (Stanislav Terliakov, PR 18465)
Fix attribute type resolution with multiple inheritance (Stanislav Terliakov, PR 18415)
Improve security of our GitHub Actions (sobolevn, PR 18413)
Unwrap type[Union[...]] when solving type variable constraints (Stanislav Terliakov, PR 18266)
Allow Any to match sequence patterns in match/case (Stanislav Terliakov, PR 18448)
Fix parent generics mapping when overriding generic attribute with property (Stanislav Terliakov, PR 18441)
Add dedicated error code for explicit Any (Shantanu, PR 18398)
Reject invalid ParamSpec locations (Stanislav Terliakov, PR 18278)
Stop suggesting stubs that have been removed from typeshed (Shantanu, PR 18373)
Allow inverting --local-partial-types (Shantanu, PR 18377)
Allow to use Final and ClassVar after Python 3.13 (정승원, PR 18358)
Update suggestions to include latest stubs in typeshed (Shantanu, PR 18366)
Fix --install-types masking failure details (wyattscarpenter, PR 17485)
Reject promotions when checking against protocols (Christoph Tyralla, PR 18360)
Don't erase type object arguments in diagnostics (Shantanu, PR 18352)
Clarify status in dmypy status output (Kcornw, PR 18331)
Disallow no-argument generic aliases when using PEP 613 explicit aliases (Brian Schubert, PR 18173)
Suppress errors for unreachable branches in conditional expressions (Brian Schubert, PR 18295)
Do not allow ClassVar and Final in TypedDict and NamedTuple (sobolevn, PR 18281)
Report error if not enough or too many types provided to TypeAliasType (bzoracler, PR 18308)
Use more precise context for TypedDict plugin errors (Brian Schubert, PR 18293)
Use more precise context for invalid type argument errors (Brian Schubert, PR 18290)
Do not allow type[] to contain Literal types (sobolevn, PR 18276)
Allow bytearray/bytes comparisons with --strict-bytes (Jukka Lehtosalo, PR 18255)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Advait Dixit
Ali Hamdan
Brian Schubert
bzoracler
Cameron Matsui
Christoph Tyralla
Gareth
Ivan Levkivskyi
Jukka Lehtosalo
Kcornw
Marc Mueller
Mikhail f. Shiryaev
Shantanu
sobolevn
Stanislav Terliakov
Stephen Morton
Valentin Stanciu
Viktor Szépe
wyattscarpenter
정승원

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.14

We’ve just uploaded mypy 1.14 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Change to Enum Membership Semantics

As per the updated typing specification for enums, enum members must be left unannotated.

class Pet(Enum):
    CAT = 1  # Member attribute
    DOG = 2  # Member attribute

    # New error: Enum members must be left unannotated
    WOLF: int = 3

    species: str  # Considered a non-member attribute

In particular, the specification change can result in issues in type stubs (.pyi files), since historically it was common to leave the value absent:

# In a type stub (.pyi file)

class Pet(Enum):
    # Change in semantics: previously considered members,
    # now non-member attributes
    CAT: int
    DOG: int

    # Mypy will now issue a warning if it detects this
    # situation in type stubs:
    # > Detected enum "Pet" in a type stub with zero
    # > members. There is a chance this is due to a recent
    # > change in the semantics of enum membership. If so,
    # > use `member = value` to mark an enum member,
    # > instead of `member: type`

class Pet(Enum):
    # As per the specification, you should now do one of
    # the following:
    DOG = 1  # Member attribute with value 1 and known type
    WOLF = cast(int, ...)  # Member attribute with unknown
                           # value but known type
    LION = ...  # Member attribute with unknown value and
                # # unknown type

Contributed by Terence Honles (PR 17207) and Shantanu Jain (PR 18068).

Support for @deprecated Decorator (PEP 702)

Mypy can now issue errors or notes when code imports a deprecated feature explicitly with a from mod import depr statement, or uses a deprecated feature imported otherwise or defined locally. Features are considered deprecated when decorated with warnings.deprecated, as specified in PEP 702.

You can enable the error code via --enable-error-code=deprecated on the mypy command line or enable_error_code = deprecated in the mypy config file. Use the command line flag --report-deprecated-as-note or config file option report_deprecated_as_note=True to turn all such errors into notes.

Deprecation errors will be enabled by default in a future mypy version.

This feature was contributed by Christoph Tyralla.

List of changes:

Add basic support for PEP 702 (@deprecated) (Christoph Tyralla, PR 17476)
Support descriptors with @deprecated (Christoph Tyralla, PR 18090)
Make “deprecated” note an error, disabled by default (Valentin Stanciu, PR 18192)
Consider all possible type positions with @deprecated (Christoph Tyralla, PR 17926)
Improve the handling of explicit type annotations in assignment statements with @deprecated (Christoph Tyralla, PR 17899)

Optionally Analyzing Untyped Modules

Mypy normally doesn't analyze imports from third-party modules (installed using pip, for example) if there are no stubs or a py.typed marker file. To force mypy to analyze these imports, you can now use the --follow-untyped-imports flag or set the follow_untyped_imports config file option to True. This can be set either in the global section of your mypy config file, or individually on a per-module basis.

This feature was contributed by Jannick Kremer.

List of changes:

Implement flag to allow type checking of untyped modules (Jannick Kremer, PR 17712)
Warn about --follow-untyped-imports (Shantanu, PR 18249)

Support New Style Type Variable Defaults (PEP 696)

Mypy now supports type variable defaults using the new syntax described in PEP 696, which was introduced in Python 3.13. Example:

@dataclass
class Box[T = int]:  # Set default for "T"
    value: T | None = None

reveal_type(Box())                      # type is Box[int], since it's the default
reveal_type(Box(value="Hello World!"))  # type is Box[str]

This feature was contributed by Marc Mueller (PR 17985).

Improved For Loop Index Variable Type Narrowing

Mypy now preserves the literal type of for loop index variables, to support TypedDict lookups. Example:

from typing import TypedDict

class X(TypedDict):
    hourly: int
    daily: int

def func(x: X) -> int:
    s = 0
    for var in ("hourly", "daily"):
        # "Union[Literal['hourly']?, Literal['daily']?]"
        reveal_type(var)

        # x[var] no longer triggers a literal-required error
        s += x[var]
    return s

This was contributed by Marc Mueller (PR 18014).

Mypyc Improvements

Document optimized bytes operations and additional str operations (Jukka Lehtosalo, PR 18242)
Add primitives and specialization for ord() (Jukka Lehtosalo, PR 18240)
Optimize str.encode with specializations for common used encodings (Valentin Stanciu, PR 18232)
Fix fall back to generic operation for staticmethod and classmethod (Advait Dixit, PR 18228)
Support unicode surrogates in string literals (Jukka Lehtosalo, PR 18209)
Fix index variable in for loop with builtins.enumerate (Advait Dixit, PR 18202)
Fix check for enum classes (Advait Dixit, PR 18178)
Fix loading type from imported modules (Advait Dixit, PR 18158)
Fix initializers of final attributes in class body (Jared Hance, PR 18031)
Fix name generation for modules with similar full names (aatle, PR 18001)
Fix relative imports in __init__.py (Shantanu, PR 17979)
Optimize dunder methods (jairov4, PR 17934)
Replace deprecated _PyDict_GetItemStringWithError (Marc Mueller, PR 17930)
Fix wheel build for cp313-win (Marc Mueller, PR 17941)
Use public PyGen_GetCode instead of vendored implementation (Marc Mueller, PR 17931)
Optimize calls to final classes (jairov4, PR 17886)
Support ellipsis (...) expressions in class bodies (Newbyte, PR 17923)
Sync pythoncapi_compat.h (Marc Mueller, PR 17929)
Add runtests.py mypyc-fast for running fast mypyc tests (Jukka Lehtosalo, PR 17906)

Stubgen Improvements

Do not include mypy generated symbols (Ali Hamdan, PR 18137)
Fix FunctionContext.fullname for nested classes (Chad Dombrova, PR 17963)
Add flagfile support (Ruslan Sayfutdinov, PR 18061)
Add support for PEP 695 and PEP 696 syntax (Ali Hamdan, PR 18054)

Stubtest Improvements

Allow the use of --show-traceback and --pdb with stubtest (Stephen Morton, PR 18037)
Verify __all__ exists in stub (Sebastian Rittau, PR 18005)
Stop telling people to use double underscores (Jelle Zijlstra, PR 17897)

Documentation Updates

Update config file documentation (sobolevn, PR 18103)
Improve contributor documentation for Windows (ag-tafe, PR 18097)
Correct note about --disallow-any-generics flag in documentation (Abel Sen, PR 18055)
Further caution against --follow-imports=skip (Shantanu, PR 18048)
Fix the edit page button link in documentation (Kanishk Pachauri, PR 17933)

Other Notables Fixes and Improvements

Allow enum members to have type objects as values (Jukka Lehtosalo, PR 19160)
Show Protocol __call__ for arguments with incompatible types (MechanicalConstruct, PR 18214)
Make join and meet symmetric with strict_optional (MechanicalConstruct, PR 18227)
Preserve block unreachablility when checking function definitions with constrained TypeVars (Brian Schubert, PR 18217)
Do not include non-init fields in the synthesized __replace__ method for dataclasses (Victorien, PR 18221)
Disallow TypeVar constraints parameterized by type variables (Brian Schubert, PR 18186)
Always complain about invalid varargs and varkwargs (Shantanu, PR 18207)
Set default strict_optional state to True (Shantanu, PR 18198)
Preserve type variable default None in type alias (Sukhorosov Aleksey, PR 18197)
Add checks for invalid usage of continue/break/return in except* block (coldwolverine, PR 18132)
Do not consider bare TypeVar not overlapping with None for reachability analysis (Stanislav Terliakov, PR 18138)
Special case types.DynamicClassAttribute as property-like (Stephen Morton, PR 18150)
Disallow bare ParamSpec in type aliases (Brian Schubert, PR 18174)
Move long_description metadata to pyproject.toml (Marc Mueller, PR 18172)
Support ==-based narrowing of Optional (Christoph Tyralla, PR 18163)
Allow TypedDict assignment of Required item to NotRequired ReadOnly item (Brian Schubert, PR 18164)
Allow nesting of Annotated with TypedDict special forms inside TypedDicts (Brian Schubert, PR 18165)
Infer generic type arguments for slice expressions (Brian Schubert, PR 18160)
Fix checking of match sequence pattern against bounded type variables (Brian Schubert, PR 18091)
Fix incorrect truthyness for Enum types and literals (David Salvisberg, PR 17337)
Move static project metadata to pyproject.toml (Marc Mueller, PR 18146)
Fallback to stdlib json if integer exceeds 64-bit range (q0w, PR 18148)
Fix ‘or’ pattern structural matching exhaustiveness (yihong, PR 18119)
Fix type inference of positional parameter in class pattern involving builtin subtype (Brian Schubert, PR 18141)
Fix [override] error with no line number when argument node has no line number (Brian Schubert, PR 18122)
Fix some dmypy crashes (Ivan Levkivskyi, PR 18098)
Fix subtyping between instance type and overloaded (Shantanu, PR 18102)
Clean up new_semantic_analyzer config (Shantanu, PR 18071)
Issue warning for enum with no members in stub (Shantanu, PR 18068)
Fix enum attributes are not members (Terence Honles, PR 17207)
Fix crash when checking slice expression with step 0 in tuple index (Brian Schubert, PR 18063)
Allow union-with-callable attributes to be overridden by methods (Brian Schubert, PR 18018)
Emit [mutable-override] for covariant override of attribute with method (Brian Schubert, PR 18058)
Support ParamSpec mapping with functools.partial (Stanislav Terliakov, PR 17355)
Fix approved stub ignore, remove normpath (Shantanu, PR 18045)
Make disallow-any-unimported flag invertible (Séamus Ó Ceanainn, PR 18030)
Filter to possible package paths before trying to resolve a module (falsedrow, PR 18038)
Fix overlap check for ParamSpec types (Jukka Lehtosalo, PR 18040)
Do not prioritize ParamSpec signatures during overload resolution (Stanislav Terliakov, PR 18033)
Fix ternary union for literals (Ivan Levkivskyi, PR 18023)
Fix compatibility checks for conditional function definitions using decorators (Brian Schubert, PR 18020)
TypeGuard should be bool not Any when matching TypeVar (Evgeniy Slobodkin, PR 17145)
Fix convert-cache tool (Shantanu, PR 17974)
Fix generator comprehension with mypyc (Shantanu, PR 17969)
Fix crash issue when using shadowfile with pretty (Max Chang, PR 17894)
Fix multiple nested classes with new generics syntax (Max Chang, PR 17820)
Better error for mypy -p package without py.typed (Joe Gordon, PR 17908)
Emit error for raise NotImplemented (Brian Schubert, PR 17890)
Add is_lvalue attribute to AttributeContext (Brian Schubert, PR 17881)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

aatle
Abel Sen
Advait Dixit
ag-tafe
Alex Waygood
Ali Hamdan
Brian Schubert
Carlton Gibson
Chad Dombrova
Chelsea Durazo
chiri
Christoph Tyralla
coldwolverine
David Salvisberg
Ekin Dursun
Evgeniy Slobodkin
falsedrow
Gaurav Giri
Ihor
Ivan Levkivskyi
jairov4
Jannick Kremer
Jared Hance
Jelle Zijlstra
jianghuyiyuan
Joe Gordon
John Doknjas
Jukka Lehtosalo
Kanishk Pachauri
Marc Mueller
Max Chang
MechanicalConstruct
Newbyte
q0w
Ruslan Sayfutdinov
Sebastian Rittau
Shantanu
sobolevn
Stanislav Terliakov
Stephen Morton
Sukhorosov Aleksey
Séamus Ó Ceanainn
Terence Honles
Valentin Stanciu
vasiliy
Victorien
yihong

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.13

We’ve just uploaded mypy 1.13 to the Python Package Index (PyPI). Mypy is a static type checker for Python. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Note that unlike typical releases, Mypy 1.13 does not have any changes to type checking semantics from 1.12.1.

Improved Performance

Mypy 1.13 contains several performance improvements. Users can expect mypy to be 5-20% faster. In environments with long search paths (such as environments using many editable installs), mypy can be significantly faster, e.g. 2.2x faster in the use case targeted by these improvements.

Mypy 1.13 allows use of the orjson library for handling the cache instead of the stdlib json, for improved performance. You can ensure the presence of orjson using the faster-cache extra:

python3 -m pip install -U mypy[faster-cache]

Mypy may depend on orjson by default in the future.

These improvements were contributed by Shantanu.

List of changes:

Significantly speed up file handling error paths (Shantanu, PR 17920)
Use fast path in modulefinder more often (Shantanu, PR 17950)
Let mypyc optimise os.path.join (Shantanu, PR 17949)
Make is_sub_path faster (Shantanu, PR 17962)
Speed up stubs suggestions (Shantanu, PR 17965)
Use sha1 for hashing (Shantanu, PR 17953)
Use orjson instead of json, when available (Shantanu, PR 17955)
Add faster-cache extra, test in CI (Shantanu, PR 17978)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Shantanu Jain
Jukka Lehtosalo

Mypy 1.12

We’ve just uploaded mypy 1.12 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Support Python 3.12 Syntax for Generics (PEP 695)

Support for the new type parameter syntax introduced in Python 3.12 is now enabled by default, documented, and no longer experimental. It was available through a feature flag in mypy 1.11 as an experimental feature.

This example demonstrates the new syntax:

# Generic function
def f[T](x: T) -> T: ...

reveal_type(f(1))  # Revealed type is 'int'

# Generic class
class C[T]:
    def __init__(self, x: T) -> None:
       self.x = x

c = C('a')
reveal_type(c.x)  # Revealed type is 'str'

# Type alias
type A[T] = C[list[T]]

For more information, refer to the documentation.

These improvements are included:

Document Python 3.12 type parameter syntax (Jukka Lehtosalo, PR 17816)
Further documentation updates (Jukka Lehtosalo, PR 17826)
Allow Self return types with contravariance (Jukka Lehtosalo, PR 17786)
Enable new type parameter syntax by default (Jukka Lehtosalo, PR 17798)
Generate error if new-style type alias used as base class (Jukka Lehtosalo, PR 17789)
Inherit variance if base class has explicit variance (Jukka Lehtosalo, PR 17787)
Fix crash on invalid type var reference (Jukka Lehtosalo, PR 17788)
Fix covariance of frozen dataclasses (Jukka Lehtosalo, PR 17783)
Allow covariance with attribute that has “_” name prefix (Jukka Lehtosalo, PR 17782)
Support Annotated[...] in new-style type aliases (Jukka Lehtosalo, PR 17777)
Fix nested generic classes (Jukka Lehtosalo, PR 17776)
Add detection and error reporting for the use of incorrect expressions within the scope of a type parameter and a type alias (Kirill Podoprigora, PR 17560)

Basic Support for Python 3.13

This release adds partial support for Python 3.13 features and compiled binaries for Python 3.13. Mypyc now also supports Python 3.13.

In particular, these features are supported:

Various new stdlib features and changes (through typeshed stub improvements)
typing.ReadOnly (see below for more)
typing.TypeIs (added in mypy 1.10, PEP 742)
Type parameter defaults when using the legacy syntax (PEP 696)

These features are not supported yet:

warnings.deprecated (PEP 702)
Type parameter defaults when using Python 3.12 type parameter syntax

Mypyc Support for Python 3.13

Mypyc now supports Python 3.13. This was contributed by Marc Mueller, with additional fixes by Jukka Lehtosalo. Free threaded Python 3.13 builds are not supported yet.

List of changes:

Add additional includes for Python 3.13 (Marc Mueller, PR 17506)
Add another include for Python 3.13 (Marc Mueller, PR 17509)
Fix ManagedDict functions for Python 3.13 (Marc Mueller, PR 17507)
Update mypyc test output for Python 3.13 (Marc Mueller, PR 17508)
Fix PyUnicode functions for Python 3.13 (Marc Mueller, PR 17504)
Fix _PyObject_LookupAttrId for Python 3.13 (Marc Mueller, PR 17505)
Fix _PyList_Extend for Python 3.13 (Marc Mueller, PR 17503)
Fix gen_is_coroutine for Python 3.13 (Marc Mueller, PR 17501)
Fix _PyObject_FastCall for Python 3.13 (Marc Mueller, PR 17502)
Avoid uses of _PyObject_CallMethodOneArg on 3.13 (Jukka Lehtosalo, PR 17526)
Don't rely on _PyType_CalculateMetaclass on 3.13 (Jukka Lehtosalo, PR 17525)
Don't use _PyUnicode_FastCopyCharacters on 3.13 (Jukka Lehtosalo, PR 17524)
Don‘t use _PyUnicode_EQ on 3.13, as it’s no longer exported (Jukka Lehtosalo, PR 17523)

Inferring Unions for Conditional Expressions

Mypy now always tries to infer a union type for a conditional expression if left and right operand types are different. This results in more precise inferred types and lets mypy detect more issues. Example:

s = "foo" if cond() else 1
# Type of "s" is now "str | int" (it used to be "object")

Notably, if one of the operands has type Any, the type of a conditional expression is now <type> | Any. Previously the inferred type was just Any. The new type essentially indicates that the value can be of type <type>, and potentially of some (unknown) type. Most operations performed on the result must also be valid for <type>. Example where this is relevant:

from typing import Any

def func(a: Any, b: bool) -> None:
    x = a if b else None
    # Type of x is "Any | None"
    print(x.y)  # Error: None has no attribute "y"

This feature was contributed by Ivan Levkivskyi (PR 17427).

ReadOnly Support for TypedDict (PEP 705)

You can now use typing.ReadOnly to specity TypedDict items as read-only (PEP 705):

from typing import TypedDict

# Or "from typing ..." on Python 3.13
from typing_extensions import ReadOnly

class TD(TypedDict):
    a: int
    b: ReadOnly[int]

d: TD = {"a": 1, "b": 2}
d["a"] = 3  # OK
d["b"] = 5  # Error: "b" is ReadOnly

This feature was contributed by Nikita Sobolev (PR 17644).

Python 3.8 End of Life Approaching

We are planning to drop support for Python 3.8 in the next mypy feature release or the one after that. Python 3.8 reaches end of life in October 2024.

Planned Changes to Defaults

We are planning to enable --local-partial-types by default in mypy 2.0. This will often require at least minor code changes. This option is implicitly enabled by mypy daemon, so this makes the behavior of daemon and non-daemon modes consistent.

We recommend that mypy users start using local partial types soon (or to explicitly disable them) to prepare for the change.

This can also be configured in a mypy configuration file:

local_partial_types = True

For more information, refer to the documentation.

Documentation Updates

Mypy documentation now uses modern syntax variants and imports in many examples. Some examples no longer work on Python 3.8, which is the earliest Python version that mypy supports.

Notably, Iterable and other protocols/ABCs are imported from collections.abc instead of typing:

from collections.abc import Iterable, Callable

Examples also avoid the upper-case aliases to built-in types: list[str] is used instead of List[str]. The X | Y union type syntax introduced in Python 3.10 is also now prevalent.

List of documentation updates:

Document --output=json CLI option (Edgar Ramírez Mondragón, PR 17611)
Update various references to deprecated type aliases in docs (Jukka Lehtosalo, PR 17829)
Make “X | Y” union syntax more prominent in documentation (Jukka Lehtosalo, PR 17835)
Discuss upper bounds before self types in documentation (Jukka Lehtosalo, PR 17827)
Make changelog visible in mypy documentation (quinn-sasha, PR 17742)
List all incomplete features in --enable-incomplete-feature docs (sobolevn, PR 17633)
Remove the explicit setting of a pygments theme (Pradyun Gedam, PR 17571)
Document ReadOnly with TypedDict (Jukka Lehtosalo, PR 17905)
Document TypeIs (Chelsea Durazo, PR 17821)

Experimental Inline TypedDict Syntax

Mypy now supports a non-standard, experimental syntax for defining anonymous TypedDicts. Example:

def func(n: str, y: int) -> {"name": str, "year": int}:
    return {"name": n, "year": y}

The feature is disabled by default. Use --enable-incomplete-feature=InlineTypedDict to enable it. We might remove this feature in a future release.

This feature was contributed by Ivan Levkivskyi (PR 17457).

Stubgen Improvements

Fix crash on literal class-level keywords (sobolevn, PR 17663)
Stubgen add --version (sobolevn, PR 17662)
Fix stubgen --no-analysis/--parse-only docs (sobolevn, PR 17632)
Include keyword only args when generating signatures in stubgenc (Eric Mark Martin, PR 17448)
Add support for detecting Literal types when extracting types from docstrings (Michael Carlstrom, PR 17441)
Use Generator type var defaults (Sebastian Rittau, PR 17670)

Stubtest Improvements

Add support for cached_property (Ali Hamdan, PR 17626)
Add enable_incomplete_feature validation to stubtest (sobolevn, PR 17635)
Fix error code handling in stubtest with --mypy-config-file (sobolevn, PR 17629)

Other Notables Fixes and Improvements

Report error if using unsupported type parameter defaults (Jukka Lehtosalo, PR 17876)
Fix re-processing cross-reference in mypy daemon when node kind changes (Ivan Levkivskyi, PR 17883)
Don't use equality to narrow when value is IntEnum/StrEnum (Jukka Lehtosalo, PR 17866)
Don't consider None vs IntEnum comparison ambiguous (Jukka Lehtosalo, PR 17877)
Fix narrowing of IntEnum and StrEnum types (Jukka Lehtosalo, PR 17874)
Filter overload items based on self type during type inference (Jukka Lehtosalo, PR 17873)
Enable negative narrowing of union TypeVar upper bounds (Brian Schubert, PR 17850)
Fix issue with member expression formatting (Brian Schubert, PR 17848)
Avoid type size explosion when expanding types (Jukka Lehtosalo, PR 17842)
Fix negative narrowing of tuples in match statement (Brian Schubert, PR 17817)
Narrow falsey str/bytes/int to literal type (Brian Schubert, PR 17818)
Test against latest Python 3.13, make testing 3.14 easy (Shantanu, PR 17812)
Reject ParamSpec-typed callables calls with insufficient arguments (Stanislav Terliakov, PR 17323)
Fix crash when passing too many type arguments to generic base class accepting single ParamSpec (Brian Schubert, PR 17770)
Fix TypeVar upper bounds sometimes not being displayed in pretty callables (Brian Schubert, PR 17802)
Added error code for overlapping function signatures (Katrina Connors, PR 17597)
Check for truthy-bool in not ... unary expressions (sobolevn, PR 17773)
Add missing lines-covered and lines-valid attributes (Soubhik Kumar Mitra, PR 17738)
Fix another crash scenario with recursive tuple types (Ivan Levkivskyi, PR 17708)
Resolve TypeVar upper bounds in functools.partial (Shantanu, PR 17660)
Always reset binder when checking deferred nodes (Ivan Levkivskyi, PR 17643)
Fix crash on a callable attribute with single unpack (Ivan Levkivskyi, PR 17641)
Fix mismatched signature between checker plugin API and implementation (bzoracler, PR 17343)
Indexing a type also produces a GenericAlias (Shantanu, PR 17546)
Fix crash on self-type in callable protocol (Ivan Levkivskyi, PR 17499)
Fix crash on NamedTuple with method and error in function (Ivan Levkivskyi, PR 17498)
Add __replace__ for dataclasses in 3.13 (Max Muoto, PR 17469)
Fix help message for --no-namespace-packages (Raphael Krupinski, PR 17472)
Fix typechecking for async generators (Danny Yang, PR 17452)
Fix strict optional handling in attrs plugin (Ivan Levkivskyi, PR 17451)
Allow mixing ParamSpec and TypeVarTuple in Generic (Ivan Levkivskyi, PR 17450)
Improvements to functools.partial of types (Shantanu, PR 17898)
Make ReadOnly TypedDict items covariant (Jukka Lehtosalo, PR 17904)
Fix union callees with functools.partial (Jukka Lehtosalo, PR 17903)
Improve handling of generic functions with functools.partial (Ivan Levkivskyi, PR 17925)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Mypy 1.12.1

Fix crash when showing partially analyzed type in error message (Ivan Levkivskyi, PR 17961)
Fix iteration over union (when self type is involved) (Shantanu, PR 17976)
Fix type object with type var default in union context (Jukka Lehtosalo, PR 17991)
Revert change to os.path stubs affecting use of os.PathLike[Any] (Shantanu, PR 17995)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Ali Hamdan
Anders Kaseorg
Bénédikt Tran
Brian Schubert
bzoracler
Chelsea Durazo
Danny Yang
Edgar Ramírez Mondragón
Eric Mark Martin
InSync
Ivan Levkivskyi
Jordandev678
Katrina Connors
Kirill Podoprigora
Marc Mueller
Max Muoto
Max Murin
Michael Carlstrom
Michael I Chen
Pradyun Gedam
quinn-sasha
Raphael Krupinski
Sebastian Rittau
Shantanu
sobolevn
Soubhik Kumar Mitra
Stanislav Terliakov
wyattscarpenter

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.11

We’ve just uploaded mypy 1.11 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Support Python 3.12 Syntax for Generics (PEP 695)

Mypy now supports the new type parameter syntax introduced in Python 3.12 (PEP 695). This feature is still experimental and must be enabled with the --enable-incomplete-feature=NewGenericSyntax flag, or with enable_incomplete_feature = NewGenericSyntax in the mypy configuration file. We plan to enable this by default in the next mypy feature release.

This example demonstrates the new syntax:

# Generic function
def f[T](x: T) -> T: ...

reveal_type(f(1))  # Revealed type is 'int'

# Generic class
class C[T]:
    def __init__(self, x: T) -> None:
       self.x = x

c = C('a')
reveal_type(c.x)  # Revealed type is 'str'

# Type alias
type A[T] = C[list[T]]

This feature was contributed by Jukka Lehtosalo.

Support for `functools.partial`

Mypy now type checks uses of functools.partial. Previously mypy would accept arbitrary arguments.

This example will now produce an error:

from functools import partial

def f(a: int, b: str) -> None: ...

g = partial(f, 1)

# Argument has incompatible type "int"; expected "str"
g(11)

This feature was contributed by Shantanu (PR 16939).

Stricter Checks for Untyped Overrides

Past mypy versions didn't check if untyped methods were compatible with overridden methods. This would result in false negatives. Now mypy performs these checks when using --check-untyped-defs.

For example, this now generates an error if using --check-untyped-defs:

class Base:
    def f(self, x: int = 0) -> None: ...

class Derived(Base):
    # Signature incompatible with "Base"
    def f(self): ...

This feature was contributed by Steven Troxler (PR 17276).

Type Inference Improvements

The new polymorphic inference algorithm introduced in mypy 1.5 is now used in more situations. This improves type inference involving generic higher-order functions, in particular.

This feature was contributed by Ivan Levkivskyi (PR 17348).

Mypy now uses unions of tuple item types in certain contexts to enable more precise inferred types. Example:

for x in (1, 'x'):
    # Previously inferred as 'object'
    reveal_type(x)  # Revealed type is 'int | str'

This was also contributed by Ivan Levkivskyi (PR 17408).

Improvements to Detection of Overlapping Overloads

The details of how mypy checks if two @overload signatures are unsafely overlapping were overhauled. This both fixes some false positives, and allows mypy to detect additional unsafe signatures.

This feature was contributed by Ivan Levkivskyi (PR 17392).

Better Support for Type Hints in Expressions

Mypy now allows more expressions that evaluate to valid type annotations in all expression contexts. The inferred types of these expressions are also sometimes more precise. Previously they were often object.

This example uses a union type that includes a callable type as an expression, and it no longer generates an error:

from typing import Callable

print(Callable[[], int] | None)  # No error

This feature was contributed by Jukka Lehtosalo (PR 17404).

Mypyc Improvements

Mypyc now supports the new syntax for generics introduced in Python 3.12 (see above). Another notable improvement is significantly faster basic operations on int values.

Support Python 3.12 syntax for generic functions and classes (Jukka Lehtosalo, PR 17357)
Support Python 3.12 type alias syntax (Jukka Lehtosalo, PR 17384)
Fix ParamSpec (Shantanu, PR 17309)
Inline fast paths of integer unboxing operations (Jukka Lehtosalo, PR 17266)
Inline tagged integer arithmetic and bitwise operations (Jukka Lehtosalo, PR 17265)
Allow specifying primitives as pure (Jukka Lehtosalo, PR 17263)

Changes to Stubtest

Ignore _ios_support (Alex Waygood, PR 17270)
Improve support for Python 3.13 (Shantanu, PR 17261)

Changes to Stubgen

Gracefully handle invalid Optional and recognize aliases to PEP 604 unions (Ali Hamdan, PR 17386)
Fix for Python 3.13 (Jelle Zijlstra, PR 17290)
Preserve enum value initialisers (Shantanu, PR 17125)

Miscellaneous New Features

Add error format support and JSON output option via --output json (Tushar Sadhwani, PR 11396)
Support enum.member in Python 3.11+ (Nikita Sobolev, PR 17382)
Support enum.nonmember in Python 3.11+ (Nikita Sobolev, PR 17376)
Support namedtuple.__replace__ in Python 3.13 (Shantanu, PR 17259)
Support rename=True in collections.namedtuple (Jelle Zijlstra, PR 17247)
Add support for __spec__ (Shantanu, PR 14739)

Changes to Error Reporting

Mention --enable-incomplete-feature=NewGenericSyntax in messages (Shantanu, PR 17462)
Do not report plugin-generated methods with explicit-override (sobolevn, PR 17433)
Use and display namespaces for function type variables (Ivan Levkivskyi, PR 17311)
Fix false positive for Final local scope variable in Protocol (GiorgosPapoutsakis, PR 17308)
Use Never in more messages, use ambiguous in join (Shantanu, PR 17304)
Log full path to config file in verbose output (dexterkennedy, PR 17180)
Added [prop-decorator] code for unsupported property decorators (#14461) (Christopher Barber, PR 16571)
Suppress second error message with := and [truthy-bool] (Nikita Sobolev, PR 15941)
Generate error for assignment of functional Enum to variable of different name (Shantanu, PR 16805)
Fix error reporting on cached run after uninstallation of third party library (Shantanu, PR 17420)

Fixes for Crashes

Fix daemon crash on invalid type in TypedDict (Ivan Levkivskyi, PR 17495)
Fix crash and bugs related to partial() (Ivan Levkivskyi, PR 17423)
Fix crash when overriding with unpacked TypedDict (Ivan Levkivskyi, PR 17359)
Fix crash on TypedDict unpacking for ParamSpec (Ivan Levkivskyi, PR 17358)
Fix crash involving recursive union of tuples (Ivan Levkivskyi, PR 17353)
Fix crash on invalid callable property override (Ivan Levkivskyi, PR 17352)
Fix crash on unpacking self in NamedTuple (Ivan Levkivskyi, PR 17351)
Fix crash on recursive alias with an optional type (Ivan Levkivskyi, PR 17350)
Fix crash on type comment inside generic definitions (Bénédikt Tran, PR 16849)

Changes to Documentation

Use inline config in documentation for optional error codes (Shantanu, PR 17374)
Use lower-case generics in documentation (Seo Sanghyeon, PR 17176)
Add documentation for show-error-code-links (GiorgosPapoutsakis, PR 17144)
Update CONTRIBUTING.md to include commands for Windows (GiorgosPapoutsakis, PR 17142)

Other Notable Improvements and Fixes

Fix ParamSpec inference against TypeVarTuple (Ivan Levkivskyi, PR 17431)
Fix explicit type for partial (Ivan Levkivskyi, PR 17424)
Always allow lambda calls (Ivan Levkivskyi, PR 17430)
Fix isinstance checks with PEP 604 unions containing None (Shantanu, PR 17415)
Fix self-referential upper bound in new-style type variables (Ivan Levkivskyi, PR 17407)
Consider overlap between instances and callables (Ivan Levkivskyi, PR 17389)
Allow new-style self-types in classmethods (Ivan Levkivskyi, PR 17381)
Fix isinstance with type aliases to PEP 604 unions (Shantanu, PR 17371)
Properly handle unpacks in overlap checks (Ivan Levkivskyi, PR 17356)
Fix type application for classes with generic constructors (Ivan Levkivskyi, PR 17354)
Update typing_extensions to >=4.6.0 to fix Python 3.12 error (Ben Brown, PR 17312)
Avoid “does not return” error in lambda (Shantanu, PR 17294)
Fix bug with descriptors in non-strict-optional mode (Max Murin, PR 17293)
Don’t leak unreachability from lambda body to surrounding scope (Anders Kaseorg, PR 17287)
Fix issues with non-ASCII characters on Windows (Alexander Leopold Shon, PR 17275)
Fix for type narrowing of negative integer literals (gilesgc, PR 17256)
Fix confusion between .py and .pyi files in mypy daemon (Valentin Stanciu, PR 17245)
Fix type of tuple[X, Y] expression (urnest, PR 17235)
Don't forget that a TypedDict was wrapped in Unpack after a name-defined error occurred (Christoph Tyralla, PR 17226)
Mark annotated argument as having an explicit, not inferred type (bzoracler, PR 17217)
Don't consider Enum private attributes as enum members (Ali Hamdan, PR 17182)
Fix Literal strings containing pipe characters (Jelle Zijlstra, PR 17148)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Mypy 1.11.1

Fix RawExpressionType.accept crash with --cache-fine-grained (Anders Kaseorg, PR 17588)
Fix PEP 604 isinstance caching (Shantanu, PR 17563)
Fix typing.TypeAliasType being undefined on python < 3.12 (Nikita Sobolev, PR 17558)
Fix types.GenericAlias lookup crash (Shantanu, PR 17543)

Mypy 1.11.2

Alternative fix for a union-like literal string (Ivan Levkivskyi, PR 17639)
Unwrap TypedDict item types before storing (Ivan Levkivskyi, PR 17640)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Alexander Leopold Shon
Ali Hamdan
Anders Kaseorg
Ben Brown
Bénédikt Tran
bzoracler
Christoph Tyralla
Christopher Barber
dexterkennedy
gilesgc
GiorgosPapoutsakis
Ivan Levkivskyi
Jelle Zijlstra
Jukka Lehtosalo
Marc Mueller
Matthieu Devlin
Michael R. Crusoe
Nikita Sobolev
Seo Sanghyeon
Shantanu
sobolevn
Steven Troxler
Tadeu Manoel
Tamir Duberstein
Tushar Sadhwani
urnest
Valentin Stanciu

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.10

We’ve just uploaded mypy 1.10 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Support TypeIs (PEP 742)

Mypy now supports TypeIs (PEP 742), which allows functions to narrow the type of a value, similar to isinstance(). Unlike TypeGuard, TypeIs can narrow in both the if and else branches of an if statement:

from typing_extensions import TypeIs

def is_str(s: object) -> TypeIs[str]:
    return isinstance(s, str)

def f(o: str | int) -> None:
    if is_str(o):
        # Type of o is 'str'
        ...
    else:
        # Type of o is 'int'
        ...

TypeIs will be added to the typing module in Python 3.13, but it can be used on earlier Python versions by importing it from typing_extensions.

This feature was contributed by Jelle Zijlstra (PR 16898).

Support TypeVar Defaults (PEP 696)

PEP 696 adds support for type parameter defaults. Example:

from typing import Generic
from typing_extensions import TypeVar

T = TypeVar("T", default=int)

class C(Generic[T]):
   ...

x: C = ...
y: C[str] = ...
reveal_type(x)  # C[int], because of the default
reveal_type(y)  # C[str]

TypeVar defaults will be added to the typing module in Python 3.13, but they can be used with earlier Python releases by importing TypeVar from typing_extensions.

This feature was contributed by Marc Mueller (PR 16878 and PR 16925).

Support TypeAliasType (PEP 695)

As part of the initial steps towards implementing PEP 695, mypy now supports TypeAliasType. TypeAliasType provides a backport of the new type statement in Python 3.12.

type ListOrSet[T] = list[T] | set[T]

is equivalent to:

T = TypeVar("T")
ListOrSet = TypeAliasType("ListOrSet", list[T] | set[T], type_params=(T,))

Example of use in mypy:

from typing_extensions import TypeAliasType, TypeVar

NewUnionType = TypeAliasType("NewUnionType", int | str)
x: NewUnionType = 42
y: NewUnionType = 'a'
z: NewUnionType = object()  # error: Incompatible types in assignment (expression has type "object", variable has type "int | str")  [assignment]

T = TypeVar("T")
ListOrSet = TypeAliasType("ListOrSet", list[T] | set[T], type_params=(T,))
a: ListOrSet[int] = [1, 2]
b: ListOrSet[str] = {'a', 'b'}
c: ListOrSet[str] = 'test'  # error: Incompatible types in assignment (expression has type "str", variable has type "list[str] | set[str]")  [assignment]

TypeAliasType was added to the typing module in Python 3.12, but it can be used with earlier Python releases by importing from typing_extensions.

This feature was contributed by Ali Hamdan (PR 16926, PR 17038 and PR 17053)

Detect Additional Unsafe Uses of super()

Mypy will reject unsafe uses of super() more consistently, when the target has a trivial (empty) body. Example:

class Proto(Protocol):
    def method(self) -> int: ...

class Sub(Proto):
    def method(self) -> int:
        return super().meth()  # Error (unsafe)

This feature was contributed by Shantanu (PR 16756).

Stubgen Improvements

Preserve empty tuple annotation (Ali Hamdan, PR 16907)
Add support for PEP 570 positional-only parameters (Ali Hamdan, PR 16904)
Replace obsolete typing aliases with builtin containers (Ali Hamdan, PR 16780)
Fix generated dataclass __init__ signature (Ali Hamdan, PR 16906)

Mypyc Improvements

Provide an easier way to define IR-to-IR transforms (Jukka Lehtosalo, PR 16998)
Implement lowering pass and add primitives for int (in)equality (Jukka Lehtosalo, PR 17027)
Implement lowering for remaining tagged integer comparisons (Jukka Lehtosalo, PR 17040)
Optimize away some bool/bit registers (Jukka Lehtosalo, PR 17022)
Remangle redefined names produced by async with (Richard Si, PR 16408)
Optimize TYPE_CHECKING to False at Runtime (Srinivas Lade, PR 16263)
Fix compilation of unreachable comprehensions (Richard Si, PR 15721)
Don't crash on non-inlinable final local reads (Richard Si, PR 15719)

Documentation Improvements

Import TypedDict from typing instead of typing_extensions (Riccardo Di Maio, PR 16958)
Add missing mutable-override to section title (James Braza, PR 16886)

Error Reporting Improvements

Use lower-case generics more consistently in error messages (Jukka Lehtosalo, PR 17035)

Other Notable Changes and Fixes

Fix incorrect inferred type when accessing descriptor on union type (Matthieu Devlin, PR 16604)
Fix crash when expanding invalid Unpack in a Callable alias (Ali Hamdan, PR 17028)
Fix false positive when string formatting with string enum (roberfi, PR 16555)
Narrow individual items when matching a tuple to a sequence pattern (Loïc Simon, PR 16905)
Fix false positive from type variable within TypeGuard or TypeIs (Evgeniy Slobodkin, PR 17071)
Improve yield from inference for unions of generators (Shantanu, PR 16717)
Fix emulating hash method logic in attrs classes (Hashem, PR 17016)
Add reverted typeshed commit that uses ParamSpec for functools.wraps (Tamir Duberstein, PR 16942)
Fix type narrowing for types.EllipsisType (Shantanu, PR 17003)
Fix single item enum match type exhaustion (Oskari Lehto, PR 16966)
Improve type inference with empty collections (Marc Mueller, PR 16994)
Fix override checking for decorated property (Shantanu, PR 16856)
Fix narrowing on match with function subject (Edward Paget, PR 16503)
Allow +N within Literal[...] (Spencer Brown, PR 16910)
Experimental: Support TypedDict within type[...] (Marc Mueller, PR 16963)
Experimtental: Fix issue with TypedDict with optional keys in type[...] (Marc Mueller, PR 17068)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Mypy 1.10.1

Fix error reporting on cached run after uninstallation of third party library (Shantanu, PR 17420)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Ali Hamdan
Edward Paget
Evgeniy Slobodkin
Hashem
hesam
Hugo van Kemenade
Ihor
James Braza
Jelle Zijlstra
jhance
Jukka Lehtosalo
Loïc Simon
Marc Mueller
Matthieu Devlin
Michael R. Crusoe
Nikita Sobolev
Oskari Lehto
Riccardo Di Maio
Richard Si
roberfi
Roman Solomatin
Sam Xifaras
Shantanu
Spencer Brown
Srinivas Lade
Tamir Duberstein
youkaichao

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.9

We’ve just uploaded mypy 1.9 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Breaking Changes

Because the version of typeshed we use in mypy 1.9 doesn't support 3.7, neither does mypy 1.9. (Jared Hance, PR 16883)

We are planning to enable local partial types (enabled via the --local-partial-types flag) later this year by default. This change was announced years ago, but now it‘s finally happening. This is a major backward-incompatible change, so we’ll probably include it as part of the upcoming mypy 2.0 release. This makes daemon and non-daemon mypy runs have the same behavior by default.

Local partial types can also be enabled in the mypy config file:

local_partial_types = True

We are looking at providing a tool to make it easier to migrate projects to use --local-partial-types, but it's not yet clear whether this is practical. The migration usually involves adding some explicit type annotations to module-level and class-level variables.

Basic Support for Type Parameter Defaults (PEP 696)

This release contains new experimental support for type parameter defaults (PEP 696). Please try it out! This feature was contributed by Marc Mueller.

Since this feature will be officially introduced in the next Python feature release (3.13), you will need to import TypeVar, ParamSpec or TypeVarTuple from typing_extensions to use defaults for now.

This example adapted from the PEP defines a default for BotT:

from typing import Generic
from typing_extensions import TypeVar

class Bot: ...

BotT = TypeVar("BotT", bound=Bot, default=Bot)

class Context(Generic[BotT]):
    bot: BotT

class MyBot(Bot): ...

# type is Bot (the default)
reveal_type(Context().bot)
# type is MyBot
reveal_type(Context[MyBot]().bot)

Type-checking Improvements

Fix missing type store for overloads (Marc Mueller, PR 16803)
Fix 'WriteToConn' object has no attribute 'flush' (Charlie Denton, PR 16801)
Improve TypeAlias error messages (Marc Mueller, PR 16831)
Support narrowing unions that include type[None] (Christoph Tyralla, PR 16315)
Support TypedDict functional syntax as class base type (anniel-stripe, PR 16703)
Accept multiline quoted annotations (Shantanu, PR 16765)
Allow unary + in Literal (Jelle Zijlstra, PR 16729)
Substitute type variables in return type of static methods (Kouroche Bouchiat, PR 16670)
Consider TypeVarTuple to be invariant (Marc Mueller, PR 16759)
Add alias support to field() in attrs plugin (Nikita Sobolev, PR 16610)
Improve attrs hashability detection (Tin Tvrtković, PR 16556)

Performance Improvements

Speed up finding function type variables (Jukka Lehtosalo, PR 16562)

Documentation Updates

Document supported values for --enable-incomplete-feature in “mypy --help” (Froger David, PR 16661)
Update new type system discussion links (thomaswhaley, PR 16841)
Add missing class instantiation to cheat sheet (Aleksi Tarvainen, PR 16817)
Document how evil --no-strict-optional is (Shantanu, PR 16731)
Improve mypy daemon documentation note about local partial types (Makonnen Makonnen, PR 16782)
Fix numbering error (Stefanie Molin, PR 16838)
Various documentation improvements (Shantanu, PR 16836)

Stubtest Improvements

Ignore private function/method parameters when they are missing from the stub (private parameter names start with a single underscore and have a default) (PR 16507)
Ignore a new protocol dunder (Alex Waygood, PR 16895)
Private parameters can be omitted (Sebastian Rittau, PR 16507)
Add support for setting enum members to “...” (Jelle Zijlstra, PR 16807)
Adjust symbol table logic (Shantanu, PR 16823)
Fix posisitional-only handling in overload resolution (Shantanu, PR 16750)

Stubgen Improvements

Fix crash on star unpack of TypeVarTuple (Ali Hamdan, PR 16869)
Use PEP 604 unions everywhere (Ali Hamdan, PR 16519)
Do not ignore property deleter (Ali Hamdan, PR 16781)
Support type stub generation for staticmethod (WeilerMarcel, PR 14934)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Aleksi Tarvainen
Alex Waygood
Ali Hamdan
anniel-stripe
Charlie Denton
Christoph Tyralla
Dheeraj
Fabian Keller
Fabian Lewis
Froger David
Ihor
Jared Hance
Jelle Zijlstra
Jukka Lehtosalo
Kouroche Bouchiat
Lukas Geiger
Maarten Huijsmans
Makonnen Makonnen
Marc Mueller
Nikita Sobolev
Sebastian Rittau
Shantanu
Stefanie Molin
Stephen Morton
thomaswhaley
Tin Tvrtković
WeilerMarcel
Wesley Collin Wright
zipperer

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Mypy 1.8

We’ve just uploaded mypy 1.8 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Type-checking Improvements

Do not intersect types in isinstance checks if at least one is final (Christoph Tyralla, PR 16330)
Detect that @final class without __bool__ cannot have falsey instances (Ilya Priven, PR 16566)
Do not allow TypedDict classes with extra keywords (Nikita Sobolev, PR 16438)
Do not allow class-level keywords for NamedTuple (Nikita Sobolev, PR 16526)
Make imprecise constraints handling more robust (Ivan Levkivskyi, PR 16502)
Fix strict-optional in extending generic TypedDict (Ivan Levkivskyi, PR 16398)
Allow type ignores of PEP 695 constructs (Shantanu, PR 16608)
Enable type_check_only support for TypedDict and NamedTuple (Nikita Sobolev, PR 16469)

Performance Improvements

Add fast path to analyzing special form assignments (Jukka Lehtosalo, PR 16561)

Improvements to Error Reporting

Don't show documentation links for plugin error codes (Ivan Levkivskyi, PR 16383)
Improve error messages for super checks and add more tests (Nikita Sobolev, PR 16393)
Add error code for mutable covariant override (Ivan Levkivskyi, PR 16399)

Stubgen Improvements

Preserve simple defaults in function signatures (Ali Hamdan, PR 15355)
Include __all__ in output (Jelle Zijlstra, PR 16356)
Fix stubgen regressions with pybind11 and mypy 1.7 (Chad Dombrova, PR 16504)

Stubtest Improvements

Improve handling of unrepresentable defaults (Jelle Zijlstra, PR 16433)
Print more helpful errors if a function is missing from stub (Alex Waygood, PR 16517)
Support @type_check_only decorator (Nikita Sobolev, PR 16422)
Warn about missing __del__ (Shantanu, PR 16456)
Fix crashes with some uses of final and deprecated (Shantanu, PR 16457)

Fixes to Crashes

Fix crash with type alias to Callable[[Unpack[Tuple[Any, ...]]], Any] (Alex Waygood, PR 16541)
Fix crash on TypeGuard in __call__ (Ivan Levkivskyi, PR 16516)
Fix crash on invalid enum in method (Ivan Levkivskyi, PR 16511)
Fix crash on unimported Any in TypedDict (Ivan Levkivskyi, PR 16510)

Documentation Updates

Update soft-error-limit default value to -1 (Sveinung Gundersen, PR 16542)
Support Sphinx 7.x (Michael R. Crusoe, PR 16460)

Other Notable Changes and Fixes

Allow mypy to output a junit file with per-file results (Matthew Wright, PR 16388)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Ali Hamdan
Chad Dombrova
Christoph Tyralla
Ilya Priven
Ivan Levkivskyi
Jelle Zijlstra
Jukka Lehtosalo
Marcel Telka
Matthew Wright
Michael R. Crusoe
Nikita Sobolev
Ole Peder Brandtzæg
robjhornby
Shantanu
Sveinung Gundersen
Valentin Stanciu

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Posted by Wesley Collin Wright

Mypy 1.7

We’ve just uploaded mypy 1.7 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Using TypedDict for `**kwargs` Typing

Mypy now has support for using Unpack[...] with a TypedDict type to annotate **kwargs arguments enabled by default. Example:

# Or 'from typing_extensions import ...'
from typing import TypedDict, Unpack

class Person(TypedDict):
    name: str
    age: int

def foo(**kwargs: Unpack[Person]) -> None:
    ...

foo(name="x", age=1)  # Ok
foo(name=1)  # Error

The definition of foo above is equivalent to the one below, with keyword-only arguments name and age:

def foo(*, name: str, age: int) -> None:
    ...

Refer to PEP 692 for more information. Note that unlike in the current version of the PEP, mypy always treats signatures with Unpack[SomeTypedDict] as equivalent to their expanded forms with explicit keyword arguments, and there aren't special type checking rules for TypedDict arguments.

This was contributed by Ivan Levkivskyi back in 2022 (PR 13471).

TypeVarTuple Support Enabled (Experimental)

Mypy now has support for variadic generics (TypeVarTuple) enabled by default, as an experimental feature. Refer to PEP 646 for the details.

TypeVarTuple was implemented by Jared Hance and Ivan Levkivskyi over several mypy releases, with help from Jukka Lehtosalo.

Changes included in this release:

Fix handling of tuple type context with unpacks (Ivan Levkivskyi, PR 16444)
Handle TypeVarTuples when checking overload constraints (robjhornby, PR 16428)
Enable Unpack/TypeVarTuple support (Ivan Levkivskyi, PR 16354)
Fix crash on unpack call special-casing (Ivan Levkivskyi, PR 16381)
Some final touches for variadic types support (Ivan Levkivskyi, PR 16334)
Support PEP-646 and PEP-692 in the same callable (Ivan Levkivskyi, PR 16294)
Support new * syntax for variadic types (Ivan Levkivskyi, PR 16242)
Correctly handle variadic instances with empty arguments (Ivan Levkivskyi, PR 16238)
Correctly handle runtime type applications of variadic types (Ivan Levkivskyi, PR 16240)
Support variadic tuple packing/unpacking (Ivan Levkivskyi, PR 16205)
Better support for variadic calls and indexing (Ivan Levkivskyi, PR 16131)
Subtyping and inference of user-defined variadic types (Ivan Levkivskyi, PR 16076)
Complete type analysis of variadic types (Ivan Levkivskyi, PR 15991)

New Way of Installing Mypyc Dependencies

If you want to install package dependencies needed by mypyc (not just mypy), you should now install mypy[mypyc] instead of just mypy:

python3 -m pip install -U 'mypy[mypyc]'

Mypy has many more users than mypyc, so always installing mypyc dependencies would often bring unnecessary dependencies.

This change was contributed by Shantanu (PR 16229).

New Rules for Re-exports

Mypy no longer considers an import such as import a.b as b as an explicit re-export. The old behavior was arguably inconsistent and surprising. This may impact some stub packages, such as older versions of types-six. You can change the import to from a import b as b, if treating the import as a re-export was intentional.

This change was contributed by Anders Kaseorg (PR 14086).

Improved Type Inference

The new type inference algorithm that was recently introduced to mypy (but was not enabled by default) is now enabled by default. It improves type inference of calls to generic callables where an argument is also a generic callable, in particular. You can use --old-type-inference to disable the new behavior.

The new algorithm can (rarely) produce different error messages, different error codes, or errors reported on different lines. This is more likely in cases where generic types were used incorrectly.

The new type inference algorithm was contributed by Ivan Levkivskyi. PR 16345 enabled it by default.

Narrowing Tuple Types Using len()

Mypy now can narrow tuple types using len() checks. Example:

def f(t: tuple[int, int] | tuple[int, int, int]) -> None:
    if len(t) == 2:
        a, b = t   # Ok
    ...

This feature was contributed by Ivan Levkivskyi (PR 16237).

More Precise Tuple Lengths (Experimental)

Mypy supports experimental, more precise checking of tuple type lengths through --enable-incomplete-feature=PreciseTupleTypes. Refer to the documentation for more information.

More generally, we are planning to use --enable-incomplete-feature to introduce experimental features that would benefit from community feedback.

This feature was contributed by Ivan Levkivskyi (PR 16237).

Mypy Changelog

We now maintain a changelog in the mypy Git repository. It mirrors the contents of mypy release blog posts. We will continue to also publish release blog posts. In the future, release blog posts will be created based on the changelog near a release date.

This was contributed by Shantanu (PR 16280).

Mypy Daemon Improvements

Fix daemon crash caused by deleted submodule (Jukka Lehtosalo, PR 16370)
Fix file reloading in dmypy with --export-types (Ivan Levkivskyi, PR 16359)
Fix dmypy inspect on Windows (Ivan Levkivskyi, PR 16355)
Fix dmypy inspect for namespace packages (Ivan Levkivskyi, PR 16357)
Fix return type change to optional in generic function (Jukka Lehtosalo, PR 16342)
Fix daemon false positives related to module-level __getattr__ (Jukka Lehtosalo, PR 16292)
Fix daemon crash related to ABCs (Jukka Lehtosalo, PR 16275)
Stream dmypy output instead of dumping everything at the end (Valentin Stanciu, PR 16252)
Make sure all dmypy errors are shown (Valentin Stanciu, PR 16250)

Mypyc Improvements

Generate error on duplicate function definitions (Jukka Lehtosalo, PR 16309)
Don't crash on unreachable statements (Jukka Lehtosalo, PR 16311)
Avoid cyclic reference in nested functions (Jukka Lehtosalo, PR 16268)
Fix direct __dict__ access on inner functions in new Python (Shantanu, PR 16084)
Make tuple packing and unpacking more efficient (Jukka Lehtosalo, PR 16022)

Improvements to Error Reporting

Update starred expression error message to match CPython (Cibin Mathew, PR 16304)
Fix error code of “Maybe you forgot to use await” note (Jelle Zijlstra, PR 16203)
Use error code [unsafe-overload] for unsafe overloads, instead of [misc] (Randolf Scholz, PR 16061)
Reword the error message related to void functions (Albert Tugushev, PR 15876)
Represent bottom type as Never in messages (Shantanu, PR 15996)
Add hint for AsyncIterator incompatible return type (Ilya Priven, PR 15883)
Don't suggest stubs packages where the runtime package now ships with types (Alex Waygood, PR 16226)

Performance Improvements

Speed up type argument checking (Jukka Lehtosalo, PR 16353)
Add fast path for checking self types (Jukka Lehtosalo, PR 16352)
Cache information about whether file is typeshed file (Jukka Lehtosalo, PR 16351)
Skip expensive repr() in logging call when not needed (Jukka Lehtosalo, PR 16350)

Attrs and Dataclass Improvements

dataclass.replace: Allow transformed classes (Ilya Priven, PR 15915)
dataclass.replace: Fall through to typeshed signature (Ilya Priven, PR 15962)
Document dataclass_transform behavior (Ilya Priven, PR 16017)
attrs: Remove fields type check (Ilya Priven, PR 15983)
attrs, dataclasses: Don‘t enforce slots when base class doesn’t (Ilya Priven, PR 15976)
Fix crash on dataclass field / property collision (Nikita Sobolev, PR 16147)

Stubgen Improvements

Write stubs with utf-8 encoding (Jørgen Lind, PR 16329)
Fix missing property setter in semantic analysis mode (Ali Hamdan, PR 16303)
Unify C extension and pure python stub generators with object oriented design (Chad Dombrova, PR 15770)
Multiple fixes to the generated imports (Ali Hamdan, PR 15624)
Generate valid dataclass stubs (Ali Hamdan, PR 15625)

Fixes to Crashes

Fix incremental mode crash on TypedDict in method (Ivan Levkivskyi, PR 16364)
Fix crash on star unpack in TypedDict (Ivan Levkivskyi, PR 16116)
Fix crash on malformed TypedDict in incremental mode (Ivan Levkivskyi, PR 16115)
Fix crash with report generation on namespace packages (Shantanu, PR 16019)
Fix crash when parsing error code config with typo (Shantanu, PR 16005)
Fix __post_init__() internal error (Ilya Priven, PR 16080)

Documentation Updates

Make it easier to copy commands from README (Hamir Mahal, PR 16133)
Document and rename [overload-overlap] error code (Shantanu, PR 16074)
Document --force-uppercase-builtins and --force-union-syntax (Nikita Sobolev, PR 16049)
Document force_union_syntax and force_uppercase_builtins (Nikita Sobolev, PR 16048)
Document we're not tracking relationships between symbols (Ilya Priven, PR 16018)

Other Notable Changes and Fixes

Propagate narrowed types to lambda expressions (Ivan Levkivskyi, PR 16407)
Avoid importing from setuptools._distutils (Shantanu, PR 16348)
Delete recursive aliases flags (Ivan Levkivskyi, PR 16346)
Properly use proper subtyping for callables (Ivan Levkivskyi, PR 16343)
Use upper bound as inference fallback more consistently (Ivan Levkivskyi, PR 16344)
Add [unimported-reveal] error code (Nikita Sobolev, PR 16271)
Add |= and | operators support for TypedDict (Nikita Sobolev, PR 16249)
Clarify variance convention for Parameters (Ivan Levkivskyi, PR 16302)
Correctly recognize typing_extensions.NewType (Ganden Schaffner, PR 16298)
Fix partially defined in the case of missing type maps (Shantanu, PR 15995)
Use SPDX license identifier (Nikita Sobolev, PR 16230)
Make __qualname__ and __module__ available in class bodies (Anthony Sottile, PR 16215)
stubtest: Hint when args in stub need to be keyword-only (Alex Waygood, PR 16210)
Tuple slice should not propagate fallback (Thomas Grainger, PR 16154)
Fix cases of type object handling for overloads (Shantanu, PR 16168)
Fix walrus interaction with empty collections (Ivan Levkivskyi, PR 16197)
Use type variable bound when it appears as actual during inference (Ivan Levkivskyi, PR 16178)
Use upper bounds as fallback solutions for inference (Ivan Levkivskyi, PR 16184)
Special-case type inference of empty collections (Ivan Levkivskyi, PR 16122)
Allow TypedDict unpacking in Callable types (Ivan Levkivskyi, PR 16083)
Fix inference for overloaded __call__ with generic self (Shantanu, PR 16053)
Call dynamic class hook on generic classes (Petter Friberg, PR 16052)
Preserve implicitly exported types via attribute access (Shantanu, PR 16129)
Fix a stubtest bug (Alex Waygood)
Fix tuple[Any, ...] subtyping (Shantanu, PR 16108)
Lenient handling of trivial Callable suffixes (Ivan Levkivskyi, PR 15913)
Add add_overloaded_method_to_class helper for plugins (Nikita Sobolev, PR 16038)
Bundle misc/proper_plugin.py as a part of mypy (Nikita Sobolev, PR 16036)
Fix case Any() in match statement (DS/Charlie, PR 14479)
Make iterable logic more consistent (Shantanu, PR 16006)
Fix inference for properties with __call__ (Shantanu, PR 15926)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Albert Tugushev
Alex Waygood
Ali Hamdan
Anders Kaseorg
Anthony Sottile
Chad Dombrova
Cibin Mathew
dinaldoap
DS/Charlie
Eli Schwartz
Ganden Schaffner
Hamir Mahal
Ihor
Ikko Eltociear Ashimine
Ilya Priven
Ivan Levkivskyi
Jelle Zijlstra
Jukka Lehtosalo
Jørgen Lind
KotlinIsland
Matt Bogosian
Nikita Sobolev
Petter Friberg
Randolf Scholz
Shantanu
Thomas Grainger
Valentin Stanciu

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Posted by Jukka Lehtosalo

Mypy 1.6

Tuesday, 10 October 2023

We’ve just uploaded mypy 1.6 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Introduce Error Subcodes for Import Errors

Mypy now uses the error code import-untyped if an import targets an installed library that doesn’t support static type checking, and no stub files are available. Other invalid imports produce the import-not-found error code. They both are subcodes of the import error code, which was previously used for both kinds of import-related errors.

Use --disable-error-code=import-untyped to only ignore import errors about installed libraries without stubs. This way mypy will still report errors about typos in import statements, for example.

If you use --warn-unused-ignore or --strict, mypy will complain if you use # type: ignore[import] to ignore an import error. You are expected to use one of the more specific error codes instead. Otherwise, ignoring the import error code continues to silence both errors.

This feature was contributed by Shantanu (PR 15840, PR 14740).

Remove Support for Targeting Python 3.6 and Earlier

Running mypy with --python-version 3.6, for example, is no longer supported. Python 3.6 hasn’t been properly supported by mypy for some time now, and this makes it explicit. This was contributed by Nikita Sobolev (PR 15668).

Selective Filtering of --disallow-untyped-calls Targets

Using --disallow-untyped-calls could be annoying when using libraries with missing type information, as mypy would generate many errors about code that uses the library. Now you can use --untyped-calls-exclude=acme, for example, to disable these errors about calls targeting functions defined in the acme package. Refer to the documentation for more information.

This feature was contributed by Ivan Levkivskyi (PR 15845).

Improved Type Inference between Callable Types

Mypy now does a better job inferring type variables inside arguments of callable types. For example, this code fragment now type checks correctly:

def f(c: Callable[[T, S], None]) -> Callable[[str, T, S], None]: ...
def g(*x: int) -> None: ...

reveal_type(f(g))  # Callable[[str, int, int], None]

This was contributed by Ivan Levkivskyi (PR 15910).

Don’t Consider None and TypeVar to Overlap in Overloads

Mypy now doesn’t consider an overload item with an argument type None to overlap with a type variable:

@overload
def f(x: None) -> None: ..
@overload
def f(x: T) -> Foo[T]: ...
...

Previously mypy would generate an error about the definition of f above. This is slightly unsafe if the upper bound of T is object, since the value of the type variable could be None. We relaxed the rules a little, since this solves a common issue.

This feature was contributed by Ivan Levkivskyi (PR 15846).

Improvements to --new-type-inference

The experimental new type inference algorithm (polymorphic inference) introduced as an opt-in feature in mypy 1.5 has several improvements:

Improve transitive closure computation during constraint solving (Ivan Levkivskyi, PR 15754)
Add support for upper bounds and values with --new-type-inference (Ivan Levkivskyi, PR 15813)
Basic support for variadic types with --new-type-inference (Ivan Levkivskyi, PR 15879)
Polymorphic inference: support for parameter specifications and lambdas (Ivan Levkivskyi, PR 15837)
Invalidate cache when adding --new-type-inference (Marc Mueller, PR 16059)

Note: We are planning to enable --new-type-inference by default in mypy 1.7. Please try this out and let us know if you encounter any issues.

ParamSpec Improvements

Support self-types containing ParamSpec (Ivan Levkivskyi, PR 15903)
Allow “…” in Concatenate, and clean up ParamSpec literals (Ivan Levkivskyi, PR 15905)
Fix ParamSpec inference for callback protocols (Ivan Levkivskyi, PR 15986)
Infer ParamSpec constraint from arguments (Ivan Levkivskyi, PR 15896)
Fix crash on invalid type variable with ParamSpec (Ivan Levkivskyi, PR 15953)
Fix subtyping between ParamSpecs (Ivan Levkivskyi, PR 15892)

Stubgen Improvements

Add option to include docstrings with stubgen (chylek, PR 13284)
Add required ... initializer to NamedTuple fields with default values (Nikita Sobolev, PR 15680)

Stubtest Improvements

Fix __mypy-replace false positives (Alex Waygood, PR 15689)
Fix edge case for bytes enum subclasses (Alex Waygood, PR 15943)
Generate error if typeshed is missing modules from the stdlib (Alex Waygood, PR 15729)
Fixes to new check for missing stdlib modules (Alex Waygood, PR 15960)
Fix stubtest enum.Flag edge case (Alex Waygood, PR 15933)

Documentation Improvements

Do not advertise to create your own assert_never helper (Nikita Sobolev, PR 15947)
Fix all the missing references found within the docs (Albert Tugushev, PR 15875)
Document await-not-async error code (Shantanu, PR 15858)
Improve documentation of disabling error codes (Shantanu, PR 15841)

Other Notable Changes and Fixes

Make unsupported PEP 695 features (introduced in Python 3.12) give a reasonable error message (Shantanu, PR 16013)
Remove the --py2 command-line argument (Marc Mueller, PR 15670)
Change empty tuple from tuple[] to tuple[()] in error messages (Nikita Sobolev, PR 15783)
Fix assert_type failures when some nodes are deferred (Nikita Sobolev, PR 15920)
Generate error on unbound TypeVar with values (Nikita Sobolev, PR 15732)
Fix over-eager types-google-cloud-ndb suggestion (Shantanu, PR 15347)
Fix type narrowing of == None and in (None,) conditions (Marti Raudsepp, PR 15760)
Fix inference for attrs.fields (Shantanu, PR 15688)
Make “await in non-async function” a non-blocking error and give it an error code (Gregory Santosa, PR 15384)
Add basic support for decorated overloads (Ivan Levkivskyi, PR 15898)
Fix TypeVar regression with self types (Ivan Levkivskyi, PR 15945)
Add __match_args__ to dataclasses with no fields (Ali Hamdan, PR 15749)
Include stdout and stderr in dmypy verbose output (Valentin Stanciu, PR 15881)
Improve match narrowing and reachability analysis (Shantanu, PR 15882)
Support __bool__ with Literal in --warn-unreachable (Jannic Warken, PR 15645)
Fix inheriting from generic @frozen attrs class (Ilya Priven, PR 15700)
Correctly narrow types for tuple[type[X], ...] (Nikita Sobolev, PR 15691)
Don't flag intentionally empty generators unreachable (Ilya Priven, PR 15722)
Add tox.ini to mypy sdist (Marcel Telka, PR 15853)
Fix mypyc regression with pretty (Shantanu, PR 16124)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to Max Murin, who did most of the release manager work for this release (I just did the final steps).

Thanks to all mypy contributors who contributed to this release:

Albert Tugushev
Alex Waygood
Ali Hamdan
chylek
EXPLOSION
Gregory Santosa
Ilya Priven
Ivan Levkivskyi
Jannic Warken
KotlinIsland
Marc Mueller
Marcel Johannesmann
Marcel Telka
Mark Byrne
Marti Raudsepp
Max Murin
Nikita Sobolev
Shantanu
Valentin Stanciu

Posted by Jukka Lehtosalo

Mypy 1.5

Thursday, 10 August 2023

We’ve just uploaded mypy 1.5 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, deprecations and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Drop Support for Python 3.7

Mypy no longer supports running with Python 3.7, which has reached end-of-life. This was contributed by Shantanu (PR 15566).

Optional Check to Require Explicit @override

If you enable the explicit-override error code, mypy will generate an error if a method override doesn’t use the @typing.override decorator (as discussed in PEP 698). This way mypy will detect accidentally introduced overrides. Example:

# mypy: enable-error-code="explicit-override"

from typing_extensions import override

class C:
    def foo(self) -> None: pass
    def bar(self) -> None: pass

class D(C):
    # Error: Method "foo" is not using @override but is
    # overriding a method
    def foo(self) -> None:
        ...

    @override
    def bar(self) -> None:  # OK
        ...

You can enable the error code via --enable-error-code=explicit-override on the mypy command line or enable_error_code = explicit-override in the mypy config file.

The override decorator will be available in typing in Python 3.12, but you can also use the backport from a recent version of typing_extensions on all supported Python versions.

This feature was contributed by Marc Mueller(PR 15512).

More Flexible TypedDict Creation and Update

Mypy was previously overly strict when type checking TypedDict creation and update operations. Though these checks were often technically correct, they sometimes triggered for apparently valid code. These checks have now been relaxed by default. You can enable stricter checking by using the new --extra-checks flag.

Construction using the ** syntax is now more flexible:

from typing import TypedDict

class A(TypedDict):
    foo: int
    bar: int

class B(TypedDict):
    foo: int

a: A = {"foo": 1, "bar": 2}
b: B = {"foo": 3}
a2: A = { **a, **b}  # OK (previously an error)

You can also call update() with a TypedDict argument that contains a subset of the keys in the updated TypedDict:

a.update(b)  # OK (previously an error)

This feature was contributed by Ivan Levkivskyi (PR 15425).

Deprecated Flag: --strict-concatenate

The behavior of --strict-concatenate is now included in the new --extra-checks flag, and the old flag is deprecated.

Optionally Show Links to Error Code Documentation

If you use --show-error-code-links, mypy will add documentation links to (many) reported errors. The links are not shown for error messages that are sufficiently obvious, and they are shown once per error code only.

Example output:

a.py:1: error: Need type annotation for "foo" (hint: "x: List[<type>] = ...")  [var-annotated]
a.py:1: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-var-annotated for more info

This was contributed by Ivan Levkivskyi (PR 15449).

Consistently Avoid Type Checking Unreachable Code

If a module top level has unreachable code, mypy won’t type check the unreachable statements. This is consistent with how functions behave. The behavior of --warn-unreachable is also more consistent now.

This was contributed by Ilya Priven (PR 15386).

Experimental Improved Type Inference for Generic Functions

You can use --new-type-inference to opt into an experimental new type inference algorithm. It fixes issues when calling a generic functions with an argument that is also a generic function, in particular. This current implementation is still incomplete, but we encourage trying it out and reporting bugs if you encounter regressions. We are planning to enable the new algorithm by default in a future mypy release.

This feature was contributed by Ivan Levkivskyi (PR 15287).

Partial Support for Python 3.12

Mypy and mypyc now support running on recent Python 3.12 development versions. Not all new Python 3.12 features are supported, and we don’t ship compiled wheels for Python 3.12 yet.

Fix ast warnings for Python 3.12 (Nikita Sobolev, PR 15558)
mypyc: Fix multiple inheritance with a protocol on Python 3.12 (Jukka Lehtosalo, PR 15572)
mypyc: Fix self-compilation on Python 3.12 (Jukka Lehtosalo, PR 15582)
mypyc: Fix 3.12 issue with pickling of instances with __dict__ (Jukka Lehtosalo, PR 15574)
mypyc: Fix i16 on Python 3.12 (Jukka Lehtosalo, PR 15510)
mypyc: Fix int operations on Python 3.12 (Jukka Lehtosalo, PR 15470)
mypyc: Fix generators on Python 3.12 (Jukka Lehtosalo, PR 15472)
mypyc: Fix classes with __dict__ on 3.12 (Jukka Lehtosalo, PR 15471)
mypyc: Fix coroutines on Python 3.12 (Jukka Lehtosalo, PR 15469)
mypyc: Don‘t use _PyErr_ChainExceptions on 3.12, since it’s deprecated (Jukka Lehtosalo, PR 15468)
mypyc: Add Python 3.12 feature macro (Jukka Lehtosalo, PR 15465)

Improvements to Dataclasses

Improve signature of dataclasses.replace (Ilya Priven, PR 14849)
Fix dataclass/protocol crash on joining types (Ilya Priven, PR 15629)
Fix strict optional handling in dataclasses (Ivan Levkivskyi, PR 15571)
Support optional types for custom dataclass descriptors (Marc Mueller, PR 15628)
Add __slots__ attribute to dataclasses (Nikita Sobolev, PR 15649)
Support better __post_init__ method signature for dataclasses (Nikita Sobolev, PR 15503)

Mypyc Improvements

Support unsigned 8-bit native integer type: mypy_extensions.u8 (Jukka Lehtosalo, PR 15564)
Support signed 16-bit native integer type: mypy_extensions.i16 (Jukka Lehtosalo, PR 15464)
Define mypy_extensions.i16 in stubs (Jukka Lehtosalo, PR 15562)
Document more unsupported features and update supported features (Richard Si, PR 15524)
Fix final NamedTuple classes (Richard Si, PR 15513)
Use C99 compound literals for undefined tuple values (Jukka Lehtosalo, PR 15453)
Don't explicitly assign NULL values in setup functions (Logan Hunt, PR 15379)

Stubgen Improvements

Teach stubgen to work with complex and unary expressions (Nikita Sobolev, PR 15661)
Support ParamSpec and TypeVarTuple (Ali Hamdan, PR 15626)
Fix crash on non-str docstring (Ali Hamdan, PR 15623)

Documentation Updates

Add documentation for additional error codes (Ivan Levkivskyi, PR 15539)
Improve documentation of type narrowing (Ilya Priven, PR 15652)
Small improvements to protocol documentation (Shantanu, PR 15460)
Remove confusing instance variable example in cheat sheet (Adel Atallah, PR 15441)

Other Notable Fixes and Improvements

Constant fold additional unary and binary expressions (Richard Si, PR 15202)
Exclude the same special attributes from Protocol as CPython (Kyle Benesch, PR 15490)
Change the default value of the slots argument of attrs.define to True, to match runtime behavior (Ilya Priven, PR 15642)
Fix type of class attribute if attribute is defined in both class and metaclass (Alex Waygood, PR 14988)
Handle type the same as typing.Type in the first argument of classmethods (Erik Kemperman, PR 15297)
Fix --find-occurrences flag (Shantanu, PR 15528)
Fix error location for class patterns (Nikita Sobolev, PR 15506)
Fix re-added file with errors in mypy daemon (Ivan Levkivskyi, PR 15440)
Fix dmypy run on Windows (Ivan Levkivskyi, PR 15429)
Fix abstract and non-abstract variant error for property deleter (Shantanu, PR 15395)
Remove special casing for “cannot” in error messages (Ilya Priven, PR 15428)
Add runtime __slots__ attribute to attrs classes (Nikita Sobolev, PR 15651)
Add get_expression_type to CheckerPluginInterface (Ilya Priven, PR 15369)
Remove parameters that no longer exist from NamedTuple._make() (Alex Waygood, PR 15578)
Allow using typing.Self in __all__ with an explicit @staticmethod decorator (Erik Kemperman, PR 15353)
Fix self types in subclass methods without Self annotation (Ivan Levkivskyi, PR 15541)
Check for abstract class objects in tuples (Nikita Sobolev, PR 15366)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Adel Atallah
Alex Waygood
Ali Hamdan
Erik Kemperman
Federico Padua
Ilya Priven
Ivan Levkivskyi
Jelle Zijlstra
Jared Hance
Jukka Lehtosalo
Kyle Benesch
Logan Hunt
Marc Mueller
Nikita Sobolev
Richard Si
Shantanu
Stavros Ntentos
Valentin Stanciu

Posted by Valentin Stanciu

Mypy 1.4

Tuesday, 20 June 2023

We’ve just uploaded mypy 1.4 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

The Override Decorator

Mypy can now ensure that when renaming a method, overrides are also renamed. You can explicitly mark a method as overriding a base class method by using the @typing.override decorator (PEP 698). If the method is then renamed in the base class while the method override is not, mypy will generate an error. The decorator will be available in typing in Python 3.12, but you can also use the backport from a recent version of typing_extensions on all supported Python versions.

This feature was contributed byThomas M Kehrenberg (PR 14609).

Propagating Type Narrowing to Nested Functions

Previously, type narrowing was not propagated to nested functions because it would not be sound if the narrowed variable changed between the definition of the nested function and the call site. Mypy will now propagate the narrowed type if the variable is not assigned to after the definition of the nested function:

def outer(x: str | None = None) -> None:
    if x is None:
        x = calculate_default()
    reveal_type(x)  # "str" (narrowed)

    def nested() -> None:
        reveal_type(x)  # Now "str" (used to be "str | None")

    nested()

This may generate some new errors because asserts that were previously necessary may become tautological or no-ops.

This was contributed by Jukka Lehtosalo (PR 15133).

Narrowing Enum Values Using “==”

Mypy now allows narrowing enum types using the == operator. Previously this was only supported when using the is operator. This makes exhaustiveness checking with enum types more usable, as the requirement to use the is operator was not very intuitive. In this example mypy can detect that the developer forgot to handle the value MyEnum.C in example

from enum import Enum

class MyEnum(Enum):
    A = 0
    B = 1
    C = 2

def example(e: MyEnum) -> str:  # Error: Missing return statement
    if e == MyEnum.A:
        return 'x'
    elif e == MyEnum.B:
        return 'y'

Adding an extra elif case resolves the error:

...
def example(e: MyEnum) -> str:  # No error -- all values covered
    if e == MyEnum.A:
        return 'x'
    elif e == MyEnum.B:
        return 'y'
    elif e == MyEnum.C:
        return 'z'

This change can cause false positives in test cases that have assert statements like assert o.x == SomeEnum.X when using --strict-equality. Example:

# mypy: strict-equality

from enum import Enum

class MyEnum(Enum):
    A = 0
    B = 1

class C:
    x: MyEnum
    ...

def test_something() -> None:
    c = C(...)
    assert c.x == MyEnum.A
    c.do_something_that_changes_x()
    assert c.x == MyEnum.B  # Error: Non-overlapping equality check

These errors can be ignored using # type: ignore[comparison-overlap], or you can perform the assertion using a temporary variable as a workaround:

...
def test_something() -> None:
    ...
    x = c.x
    assert x == MyEnum.A  # Does not narrow c.x
    c.do_something_that_changes_x()
    x = c.x
    assert x == MyEnum.B  # OK

This feature was contributed by Shantanu (PR 11521).

Performance Improvements

Speed up simplification of large union types and also fix a recursive tuple crash (Shantanu, PR 15128)
Speed up union subtyping (Shantanu, PR 15104)
Don't type check most function bodies when type checking third-party library code, or generally when ignoring errors (Jukka Lehtosalo, PR 14150)

Improvements to Plugins

attrs.evolve: Support generics and unions (Ilya Konstantinov, PR 15050)
Fix ctypes plugin (Alex Waygood)

Fixes to Crashes

Fix a crash when function-scope recursive alias appears as upper bound (Ivan Levkivskyi, PR 15159)
Fix crash on follow_imports_for_stubs (Ivan Levkivskyi, PR 15407)
Fix stubtest crash in explicit init subclass (Shantanu, PR 15399)
Fix crash when indexing TypedDict with empty key (Shantanu, PR 15392)
Fix crash on NamedTuple as attribute (Ivan Levkivskyi, PR 15404)
Correctly track loop depth for nested functions/classes (Ivan Levkivskyi, PR 15403)
Fix crash on joins with recursive tuples (Ivan Levkivskyi, PR 15402)
Fix crash with custom ErrorCode subclasses (Marc Mueller, PR 15327)
Fix crash in dataclass protocol with self attribute assignment (Ivan Levkivskyi, PR 15157)
Fix crash on lambda in generic context with generic method in body (Ivan Levkivskyi, PR 15155)
Fix recursive type alias crash in make_simplified_union (Ivan Levkivskyi, PR 15216)

Improvements to Error Messages

Use lower-case built-in collection types such as list[…] instead of List[…] in errors when targeting Python 3.9+ (Max Murin, PR 15070)
Use X | Y union syntax in error messages when targeting Python 3.10+ (Omar Silva, PR 15102)
Use type instead of Type in errors when targeting Python 3.9+ (Rohit Sanjay, PR 15139)
Do not show unused-ignore errors in unreachable code, and make it a real error code (Ivan Levkivskyi, PR 15164)
Don’t limit the number of errors shown by default (Rohit Sanjay, PR 15138)
Improver message for truthy functions (madt2709, PR 15193)
Output distinct types when type names are ambiguous (teresa0605, PR 15184)
Update message about invalid exception type in try (AJ Rasmussen, PR 15131)
Add explanation if argument type is incompatible because of an unsupported numbers type (Jukka Lehtosalo, PR 15137)
Add more detail to ‘signature incompatible with supertype’ messages for non-callables (Ilya Priven, PR 15263)

Documentation Updates

Add --local-partial-types note to dmypy docs (Alan Du, PR 15259)
Update getting started docs for mypyc for Windows (Valentin Stanciu, PR 15233)
Clarify usage of callables regarding type object in docs (Viicos, PR 15079)
Clarify difference between disallow_untyped_defs and disallow_incomplete_defs (Ilya Priven, PR 15247)
Use attrs and @attrs.define in documentation and tests (Ilya Priven, PR 15152)

Mypyc Improvements

Fix unexpected TypeError for certain variables with an inferred optional type (Richard Si, PR 15206)
Inline math literals (Logan Hunt, PR 15324)
Support unpacking mappings in dict display (Richard Si, PR 15203)

Changes to Stubgen

Do not remove Generic from base classes (Ali Hamdan, PR 15316)
Support yield from statements (Ali Hamdan, PR 15271)
Fix missing total from TypedDict class (Ali Hamdan, PR 15208)
Fix call-based namedtuple omitted from class bases (Ali Hamdan, PR 14680)
Support TypedDict alternative syntax (Ali Hamdan, PR 14682)
Make stubgen respect MYPY_CACHE_DIR (Henrik Bäärnhielm, PR 14722)
Fixes and simplifications (Ali Hamdan, PR 15232)

Other Notable Fixes and Improvements

Fix nested async functions when using TypeVar value restriction (Jukka Lehtosalo, PR 14705)
Always allow returning Any from lambda (Ivan Levkivskyi, PR 15413)
Add foundation for TypeVar defaults (PEP 696) (Marc Mueller, PR 14872)
Update semantic analyzer for TypeVar defaults (PEP 696) (Marc Mueller, PR 14873)
Make dict expression inference more consistent (Ivan Levkivskyi, PR 15174)
Do not block on duplicate base classes (Nikita Sobolev, PR 15367)
Generate an error when both staticmethod and classmethod decorators are used (Juhi Chandalia, PR 15118)
Fix assert_type behaviour with literals (Carl Karsten, PR 15123)
Fix match subject ignoring redefinitions (Vincent Vanlaer, PR 15306)
Support __all__.remove (Shantanu, PR 15279)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Adrian Garcia Badaracco
AJ Rasmussen
Alan Du
Alex Waygood
Ali Hamdan
Carl Karsten
dosisod
Ethan Smith
Gregory Santosa
Heather White
Henrik Bäärnhielm
Ilya Konstantinov
Ilya Priven
Ivan Levkivskyi
Juhi Chandalia
Jukka Lehtosalo
Logan Hunt
madt2709
Marc Mueller
Max Murin
Nikita Sobolev
Omar Silva
Özgür
Richard Si
Rohit Sanjay
Shantanu
teresa0605
Thomas M Kehrenberg
Tin Tvrtković
Tushar Sadhwani
Valentin Stanciu
Viicos
Vincent Vanlaer
Wesley Collin Wright
William Santosa
yaegassy

I’d also like to thank my employer, Dropbox, for supporting mypy development.

Posted by Jared Hance

Mypy 1.3

Wednesday, 10 May 2023

We’ve just uploaded mypy 1.3 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Performance Improvements

Improve performance of union subtyping (Shantanu, PR 15104)
Add negative subtype caches (Ivan Levkivskyi, PR 14884)

Stub Tooling Improvements

Stubtest: Check that the stub is abstract if the runtime is, even when the stub is an overloaded method (Alex Waygood, PR 14955)
Stubtest: Verify stub methods or properties are decorated with @final if they are decorated with @final at runtime (Alex Waygood, PR 14951)
Stubtest: Fix stubtest false positives with TypedDicts at runtime (Alex Waygood, PR 14984)
Stubgen: Support @functools.cached_property (Nikita Sobolev, PR 14981)
Improvements to stubgenc (Chad Dombrova, PR 14564)

Improvements to attrs

Add support for converters with TypeVars on generic attrs classes (Chad Dombrova, PR 14908)
Fix attrs.evolve on bound TypeVar (Ilya Konstantinov, PR 15022)

Documentation Updates

Improve async documentation (Shantanu, PR 14973)
Improvements to cheat sheet (Shantanu, PR 14972)
Add documentation for bytes formatting error code (Shantanu, PR 14971)
Convert insecure links to use HTTPS (Marti Raudsepp, PR 14974)
Also mention overloads in async iterator documentation (Shantanu, PR 14998)
stubtest: Improve allowlist documentation (Shantanu, PR 15008)
Clarify “Using types... but not at runtime” (Jon Shea, PR 15029)
Fix alignment of cheat sheet example (Ondřej Cvacho, PR 15039)
Fix error for callback protocol matching against callable type object (Shantanu, PR 15042)

Error Reporting Improvements

Improve bytes formatting error (Shantanu, PR 14959)

Mypyc Improvements

Fix unions of bools and ints (Tomer Chachamu, PR 15066)

Other Fixes and Improvements

Fix narrowing union types that include Self with isinstance (Christoph Tyralla, PR 14923)
Allow objects matching SupportsKeysAndGetItem to be unpacked (Bryan Forbes, PR 14990)
Check type guard validity for staticmethods (EXPLOSION, PR 14953)
Fix sys.platform when cross-compiling with emscripten (Ethan Smith, PR 14888)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Amin Alaee
Bryan Forbes
Chad Dombrova
Charlie Denton
Christoph Tyralla
dosisod
Ethan Smith
EXPLOSION
Ilya Konstantinov
Ivan Levkivskyi
Jon Shea
Jukka Lehtosalo
KotlinIsland
Marti Raudsepp
Nikita Sobolev
Ondřej Cvacho
Shantanu
sobolevn
Tomer Chachamu
Yaroslav Halchenko

Posted by Wesley Collin Wright.

Mypy 1.2

Thursday, 6 April 2023

We’ve just uploaded mypy 1.2 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Improvements to Dataclass Transforms

Support implicit default for “init” parameter in field specifiers (Wesley Collin Wright and Jukka Lehtosalo, PR 15010)
Support descriptors in dataclass transform (Jukka Lehtosalo, PR 15006)
Fix frozen_default in incremental mode (Wesley Collin Wright)
Fix frozen behavior for base classes with direct metaclasses (Wesley Collin Wright, PR 14878)

Mypyc: Native Floats

Mypyc now uses a native, unboxed representation for values of type float. Previously these were heap-allocated Python objects. Native floats are faster and use less memory. Code that uses floating-point operations heavily can be several times faster when using native floats.

Various float operations and math functions also now have optimized implementations. Refer to the documentation for a full list.

This can change the behavior of existing code that uses subclasses of float. When assigning an instance of a subclass of float to a variable with the float type, it gets implicitly converted to a float instance when compiled:

from lib import MyFloat  # MyFloat ia a subclass of "float"

def example() -> None:
    x = MyFloat(1.5)
    y: float = x  # Implicit conversion from MyFloat to float
    print(type(y))  # float, not MyFloat

Previously, implicit conversions were applied to int subclasses but not float subclasses.

Also, int values can no longer be assigned to a variable with type float in compiled code, since these types now have incompatible representations. An explicit conversion is required:

def example(n: int) -> None:
    a: float = 1  # Error: cannot assign "int" to "float"
    b: float = 1.0  # OK
    c: float = n  # Error
    d: float = float(n)  # OK

This restriction only applies to assignments, since they could otherwise narrow down the type of a variable from float to int. int values can still be implicitly converted to float when passed as arguments to functions that expect float values.

Note that mypyc still doesn’t support arrays of unboxed float values. Using list[float] involves heap-allocated float objects, since list can only store boxed values. Support for efficient floating point arrays is one of the next major planned mypyc features.

Related changes:

Use a native unboxed representation for floats (Jukka Lehtosalo, PR 14880)
Document native floats and integers (Jukka Lehtosalo, PR 14927)
Fixes to float to int conversion (Jukka Lehtosalo, PR 14936)

Mypyc: Native Integers

Mypyc now supports signed 32-bit and 64-bit integer types in addition to the arbitrary-precision int type. You can use the types mypy_extensions.i32 and mypy_extensions.i64 to speed up code that uses integer operations heavily.

Simple example:

from mypy_extensions import i64

def inc(x: i64) -> i64:
    return x + 1

Refer to the documentation for more information. This feature was contributed by Jukka Lehtosalo.

Other Mypyc Fixes and Improvements

Support iterating over a TypedDict (Richard Si, PR 14747)
Faster coercions between different tuple types (Jukka Lehtosalo, PR 14899)
Faster calls via type aliases (Jukka Lehtosalo, PR 14784)
Faster classmethod calls via cls (Jukka Lehtosalo, PR 14789)

Fixes to Crashes

Fix crash on class-level import in protocol definition (Ivan Levkivskyi, PR 14926)
Fix crash on single item union of alias (Ivan Levkivskyi, PR 14876)
Fix crash on ParamSpec in incremental mode (Ivan Levkivskyi, PR 14885)

Documentation Updates

Update adopting --strict documentation for 1.0 (Shantanu, PR 14865)
Some minor documentation tweaks (Jukka Lehtosalo, PR 14847)
Improve documentation of top level mypy: disable-error-code comment (Nikita Sobolev, PR 14810)

Error Reporting Improvements

Add error code to typing_extensions suggestion (Shantanu, PR 14881)
Add a separate error code for top-level await (Nikita Sobolev, PR 14801)
Don’t suggest two obsolete stub packages (Jelle Zijlstra, PR 14842)
Add suggestions for pandas-stubs and lxml-stubs (Shantanu, PR 14737)

Other Fixes and Improvements

Multiple inheritance considers callable objects as subtypes of functions (Christoph Tyralla, PR 14855)
stubtest: Respect @final runtime decorator and enforce it in stubs (Nikita Sobolev, PR 14922)
Fix false positives related to type[] (sterliakov, PR 14756)
Fix duplication of ParamSpec prefixes and properly substitute ParamSpecs (EXPLOSION, PR 14677)
Fix line number if __iter__ is incorrectly reported as missing (Jukka Lehtosalo, PR 14893)
Fix incompatible overrides of overloaded generics with self types (Shantanu, PR 14882)
Allow SupportsIndex in slice expressions (Shantanu, PR 14738)
Support if statements in bodies of dataclasses and classes that use dataclass_transform (Jacek Chałupka, PR 14854)
Allow iterable class objects to be unpacked (including enums) (Alex Waygood, PR 14827)
Fix narrowing for walrus expressions used in match statements (Shantanu, PR 14844)
Add signature for attr.evolve (Ilya Konstantinov, PR 14526)
Fix Any inference when unpacking iterators that don't directly inherit from typing.Iterator (Alex Waygood, PR 14821)
Fix unpack with overloaded __iter__ method (Nikita Sobolev, PR 14817)
Reduce size of JSON data in mypy cache (dosisod, PR 14808)
Improve “used before definition” checks when a local definition has the same name as a global definition (Stas Ilinskiy, PR 14517)
Honor NoReturn as __setitem__ return type to mark unreachable code (sterliakov, PR 12572)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Avasam
Christoph Tyralla
dosisod
EXPLOSION
Ilya Konstantinov
Ivan Levkivskyi
Jacek Chałupka
Jelle Zijlstra
Jukka Lehtosalo
Marc Mueller
Max Murin
Nikita Sobolev
Richard Si
Shantanu
Stas Ilinskiy
sterliakov
Wesley Collin Wright

Posted by Jukka Lehtosalo

Mypy 1.1.1

Monday, 6 March 2023

We’ve just uploaded mypy 1.1.1 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

Support for `dataclass_transform``

This release adds full support for the dataclass_transform decorator defined in PEP 681. This allows decorators, base classes, and metaclasses that generate a __init__ method or other methods based on the properties of that class (similar to dataclasses) to have those methods recognized by mypy.

This was contributed by Wesley Collin Wright.

Dedicated Error Code for Method Assignments

Mypy can’t safely check all assignments to methods (a form of monkey patching), so mypy generates an error by default. To make it easier to ignore this error, mypy now uses the new error code method-assign for this. By disabling this error code in a file or globally, mypy will no longer complain about assignments to methods if the signatures are compatible.

Mypy also supports the old error code assignment for these assignments to prevent a backward compatibility break. More generally, we can use this mechanism in the future if we wish to split or rename another existing error code without causing backward compatibility issues.

This was contributed by Ivan Levkivskyi (PR 14570).

Fixes to Crashes

Fix a crash on walrus in comprehension at class scope (Ivan Levkivskyi, PR 14556)
Fix crash related to value-constrained TypeVar (Shantanu, PR 14642)

Fixes to Cache Corruption

Fix generic TypedDict/NamedTuple caching (Ivan Levkivskyi, PR 14675)

Mypyc Fixes and Improvements

Raise “non-trait base must be first...” error less frequently (Richard Si, PR 14468)
Generate faster code for bool comparisons and arithmetic (Jukka Lehtosalo, PR 14489)
Optimize __(a)enter__/__(a)exit__ for native classes (Jared Hance, PR 14530)
Detect if attribute definition conflicts with base class/trait (Jukka Lehtosalo, PR 14535)
Support __(r)divmod__ dunders (Richard Si, PR 14613)
Support __pow__, __rpow__, and __ipow__ dunders (Richard Si, PR 14616)
Fix crash on star unpacking to underscore (Ivan Levkivskyi, PR 14624)
Fix iterating over a union of dicts (Richard Si, PR 14713)

Fixes to Detecting Undefined Names (used-before-def)

Correctly handle walrus operator (Stas Ilinskiy, PR 14646)
Handle walrus declaration in match subject correctly (Stas Ilinskiy, PR 14665)

Stubgen Improvements

Stubgen is a tool for automatically generating draft stubs for libraries.

Allow aliases below the top level (Chad Dombrova, PR 14388)
Fix crash with PEP 604 union in type variable bound (Shantanu, PR 14557)
Preserve PEP 604 unions in generated .pyi files (hamdanal, PR 14601)

Stubtest Improvements

Stubtest is a tool for testing that stubs conform to the implementations.

Update message format so that it’s easier to go to error location (Avasam, PR 14437)
Handle name-mangling edge cases better (Alex Waygood, PR 14596)

Changes to Error Reporting and Messages

Add new TypedDict error code typeddict-unknown-key (JoaquimEsteves, PR 14225)
Give arguments a more reasonable location in error messages (Max Murin, PR 14562)
In error messages, quote just the module's name (Ilya Konstantinov, PR 14567)
Improve misleading message about Enum() (Rodrigo Silva, PR 14590)
Suggest importing from typing_extensions if definition is not in typing (Shantanu, PR 14591)
Consistently use type-abstract error code (Ivan Levkivskyi, PR 14619)
Consistently use literal-required error code for TypedDicts (Ivan Levkivskyi, PR 14621)
Adjust inconsistent dataclasses plugin error messages (Wesley Collin Wright, PR 14637)
Consolidate literal bool argument error messages (Wesley Collin Wright, PR 14693)

Other Fixes and Improvements

Check that type guards accept a positional argument (EXPLOSION, PR 14238)
Fix bug with in operator used with a union of Container and Iterable (Max Murin, PR 14384)
Support protocol inference for type[T] via metaclass (Ivan Levkivskyi, PR 14554)
Allow overlapping comparisons between bytes-like types (Shantanu, PR 14658)
Fix mypy daemon documentation link in README (Ivan Levkivskyi, PR 14644)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alex Waygood
Avasam
Chad Dombrova
dosisod
EXPLOSION
hamdanal
Ilya Konstantinov
Ivan Levkivskyi
Jared Hance
JoaquimEsteves
Jukka Lehtosalo
Marc Mueller
Max Murin
Michael Lee
Michael R. Crusoe
Richard Si
Rodrigo Silva
Shantanu
Stas Ilinskiy
Wesley Collin Wright
Yilei “Dolee” Yang
Yurii Karabas

We’d also like to thank our employer, Dropbox, for funding the mypy core team.

Posted by Max Murin

Mypy 1.0

Monday, 6 February 2023

We’ve just uploaded mypy 1.0 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

New Release Versioning Scheme

Now that mypy reached 1.0, we’ll switch to a new versioning scheme. Mypy version numbers will be of form x.y.z.

Rules:

The major release number (x) is incremented if a feature release includes a significant backward incompatible change that affects a significant fraction of users.
The minor release number (y) is incremented on each feature release. Minor releases include updated stdlib stubs from typeshed.
The point release number (z) is incremented when there are fixes only.

Mypy doesn't use SemVer, since most minor releases have at least minor backward incompatible changes in typeshed, at the very least. Also, many type checking features find new legitimate issues in code. These are not considered backward incompatible changes, unless the number of new errors is very high.

Any significant backward incompatible change must be announced in the blog post for the previous feature release, before making the change. The previous release must also provide a flag to explicitly enable or disable the new behavior (whenever practical), so that users will be able to prepare for the changes and report issues. We should keep the feature flag for at least a few releases after we've switched the default.

See ”Release Process” in the mypy wiki for more details and for the most up-to-date version of the versioning scheme.

Performance Improvements

Mypy 1.0 is up to 40% faster than mypy 0.991 when type checking the Dropbox internal codebase. We also set up a daily job to measure the performance of the most recent development version of mypy to make it easier to track changes in performance.

Many optimizations contributed to this improvement:

Improve performance for errors on class with many attributes (Shantanu, PR 14379)
Speed up make_simplified_union (Jukka Lehtosalo, PR 14370)
Micro-optimize get_proper_type(s) (Jukka Lehtosalo, PR 14369)
Micro-optimize flatten_nested_unions (Jukka Lehtosalo, PR 14368)
Some semantic analyzer micro-optimizations (Jukka Lehtosalo, PR 14367)
A few miscellaneous micro-optimizations (Jukka Lehtosalo, PR 14366)
Optimization: Avoid a few uses of contextmanagers in semantic analyzer (Jukka Lehtosalo, PR 14360)
Optimization: Enable always defined attributes in Type subclasses (Jukka Lehtosalo, PR 14356)
Optimization: Remove expensive context manager in type analyzer (Jukka Lehtosalo, PR 14357)
subtypes: fast path for Union/Union subtype check (Hugues, PR 14277)
Micro-optimization: avoid Bogus[int] types that cause needless boxing (Jukka Lehtosalo, PR 14354)
Avoid slow error message logic if errors not shown to user (Jukka Lehtosalo, PR 14336)
Speed up the implementation of hasattr() checks (Jukka Lehtosalo, PR 14333)
Avoid the use of a context manager in hot code path (Jukka Lehtosalo, PR 14331)
Change various type queries into faster bool type queries (Jukka Lehtosalo, PR 14330)
Speed up recursive type check (Jukka Lehtosalo, PR 14326)
Optimize subtype checking by avoiding a nested function (Jukka Lehtosalo, PR 14325)
Optimize type parameter checks in subtype checking (Jukka Lehtosalo, PR 14324)
Speed up freshening type variables (Jukka Lehtosalo, PR 14323)
Optimize implementation of TypedDict types for **kwds (Jukka Lehtosalo, PR 14316)

Warn About Variables Used Before Definition

Mypy will now generate an error if you use a variable before it’s defined. This feature is enabled by default. By default mypy reports an error when it infers that a variable is always undefined.

y = x  # E: Name "x" is used before definition [used-before-def]
x = 0

This feature was contributed by Stas Ilinskiy.

Detect Possibly Undefined Variables (Experimental)

A new experimental possibly-undefined error code is now available that will detect variables that may be undefined:

    if b:
        x = 0
    print(x)  # Error: Name "x" may be undefined [possibly-undefined]

The error code is disabled be default, since it can generate false positives.

This feature was contributed by Stas Ilinskiy.

Support the “Self” Type

There is now a simpler syntax for declaring generic self types introduced in PEP 673: the Self type. You no longer have to define a type variable to use “self types”, and you can use them with attributes. Example from mypy documentation:

from typing import Self

class Friend:
    other: Self | None = None

    @classmethod
    def make_pair(cls) -> tuple[Self, Self]:
        a, b = cls(), cls()
        a.other = b
        b.other = a
        return a, b

class SuperFriend(Friend):
    pass

# a and b have the inferred type "SuperFriend", not "Friend"
a, b = SuperFriend.make_pair()

The feature was introduced in Python 3.11. In earlier Python versions a backport of Self is available in typing_extensions.

This was contributed by Ivan Levkivskyi (PR 14041).

Support ParamSpec in Type Aliases

ParamSpec and Concatenate can now be used in type aliases. Example:

from typing import ParamSpec, Callable

P = ParamSpec("P")
A = Callable[P, None]

def f(c: A[int, str]) -> None:
    c(1, "x")

This feature was contributed by Ivan Levkivskyi (PR 14159).

ParamSpec and Generic Self Types No Longer Experimental

Support for ParamSpec (PEP 612) and generic self types are no longer considered experimental.

Miscellaneous New Features

Minimal, partial implementation of dataclass_transform (PEP 681) (Wesley Collin Wright, PR 14523)
Add basic support for typing_extensions.TypeVar (Marc Mueller, PR 14313)
Add --debug-serialize option (Marc Mueller, PR 14155)
Constant fold initializers of final variables (Jukka Lehtosalo, PR 14283)
Enable Final instance attributes for attrs (Tin Tvrtković, PR 14232)
Allow function arguments as base classes (Ivan Levkivskyi, PR 14135)
Allow super() with mixin protocols (Ivan Levkivskyi, PR 14082)
Add type inference for dict.keys membership (Matthew Hughes, PR 13372)
Generate error for class attribute access if attribute is defined with __slots__ (Harrison McCarty, PR 14125)
Support additional attributes in callback protocols (Ivan Levkivskyi, PR 14084)

Fixes to Crashes

Fix crash on prefixed ParamSpec with forward reference (Ivan Levkivskyi, PR 14569)
Fix internal crash when resolving the same partial type twice (Shantanu, PR 14552)
Fix crash in daemon mode on new import cycle (Ivan Levkivskyi, PR 14508)
Fix crash in mypy daemon (Ivan Levkivskyi, PR 14497)
Fix crash on Any metaclass in incremental mode (Ivan Levkivskyi, PR 14495)
Fix crash in await inside comprehension outside function (Ivan Levkivskyi, PR 14486)
Fix crash in Self type on forward reference in upper bound (Ivan Levkivskyi, PR 14206)
Fix a crash when incorrect super() is used outside a method (Ivan Levkivskyi, PR 14208)
Fix crash on overriding with frozen attrs (Ivan Levkivskyi, PR 14186)
Fix incremental mode crash on generic function appearing in nested position (Ivan Levkivskyi, PR 14148)
Fix daemon crash on malformed NamedTuple (Ivan Levkivskyi, PR 14119)
Fix crash during ParamSpec inference (Ivan Levkivskyi, PR 14118)
Fix crash on nested generic callable (Ivan Levkivskyi, PR 14093)
Fix crashes with unpacking SyntaxError (Shantanu, PR 11499)
Fix crash on partial type inference within a lambda (Ivan Levkivskyi, PR 14087)
Fix crash with enums (Michael Lee, PR 14021)
Fix crash with malformed TypedDicts and disllow-any-expr (Michael Lee, PR 13963)

Error Reporting Improvements

More helpful error for missing self (Shantanu, PR 14386)
Add error-code truthy-iterable (Marc Mueller, PR 13762)
Fix pluralization in error messages (KotlinIsland, PR 14411)

Mypyc: Support Match Statement

Mypyc can now compile Python 3.10 match statements.

This was contributed by dosisod (PR 13953).

Other Mypyc Fixes and Improvements

Optimize int(x)/float(x)/complex(x) on instances of native classes (Richard Si, PR 14450)
Always emit warnings (Richard Si, PR 14451)
Faster bool and integer conversions (Jukka Lehtosalo, PR 14422)
Support attributes that override properties (Jukka Lehtosalo, PR 14377)
Precompute set literals for “in” operations and iteration (Richard Si, PR 14409)
Don't load targets with forward references while setting up non-extension class __all__ (Richard Si, PR 14401)
Compile away NewType type calls (Richard Si, PR 14398)
Improve error message for multiple inheritance (Joshua Bronson, PR 14344)
Simplify union types (Jukka Lehtosalo, PR 14363)
Fixes to union simplification (Jukka Lehtosalo, PR 14364)
Fix for typeshed changes to Collection (Shantanu, PR 13994)
Allow use of enum.Enum (Shantanu, PR 13995)
Fix compiling on Arch Linux (dosisod, PR 13978)

Documentation Improvements

Various documentation and error message tweaks (Jukka Lehtosalo, PR 14574)
Improve Generics documentation (Shantanu, PR 14587)
Improve protocols documentation (Shantanu, PR 14577)
Improve dynamic typing documentation (Shantanu, PR 14576)
Improve the Common Issues page (Shantanu, PR 14581)
Add a top-level TypedDict page (Shantanu, PR 14584)
More improvements to getting started documentation (Shantanu, PR 14572)
Move truthy-function documentation from “optional checks” to “enabled by default” (Anders Kaseorg, PR 14380)
Avoid use of implicit optional in decorator factory documentation (Tom Schraitle, PR 14156)
Clarify documentation surrounding install-types (Shantanu, PR 14003)
Improve searchability for module level type ignore errors (Shantanu, PR 14342)
Advertise mypy daemon in README (Ivan Levkivskyi, PR 14248)
Add link to error codes in README (Ivan Levkivskyi, PR 14249)
Document that report generation disables cache (Ilya Konstantinov, PR 14402)
Stop saying mypy is beta software (Ivan Levkivskyi, PR 14251)
Flycheck-mypy is deprecated, since its functionality was merged to Flycheck (Ivan Levkivskyi, PR 14247)
Update code example in “Declaring decorators” (ChristianWitzler, PR 14131)

Stubtest Improvements

Stubtest is a tool for testing that stubs conform to the implementations.

Improve error message for __all__-related errors (Alex Waygood, PR 14362)
Improve heuristics for determining whether global-namespace names are imported (Alex Waygood, PR 14270)
Catch BaseException on module imports (Shantanu, PR 14284)
Associate exported symbol error with __all__ object_path (Nikita Sobolev, PR 14217)
Add __warningregistry__ to the list of ignored module dunders (Nikita Sobolev, PR 14218)
If a default is present in the stub, check that it is correct (Jelle Zijlstra, PR 14085)

Stubgen Improvements

Stubgen is a tool for automatically generating draft stubs for libraries.

Treat dlls as C modules (Shantanu, PR 14503)

Other Notable Fixes and Improvements

Update stub suggestions based on recent typeshed changes (Alex Waygood, PR 14265)
Fix attrs protocol check with cache (Marc Mueller, PR 14558)
Fix strict equality check if operand item type has custom __eq__ (Jukka Lehtosalo, PR 14513)
Don't consider object always truthy (Jukka Lehtosalo, PR 14510)
Properly support union of TypedDicts as dict literal context (Ivan Levkivskyi, PR 14505)
Properly expand type in generic class with Self and TypeVar with values (Ivan Levkivskyi, PR 14491)
Fix recursive TypedDicts/NamedTuples defined with call syntax (Ivan Levkivskyi, PR 14488)
Fix type inference issue when a class inherits from Any (Shantanu, PR 14404)
Fix false positive on generic base class with six (Ivan Levkivskyi, PR 14478)
Don't read scripts without extensions as modules in namespace mode (Tim Geypens, PR 14335)
Fix inference for constrained type variables within unions (Christoph Tyralla, PR 14396)
Fix Unpack imported from typing (Marc Mueller, PR 14378)
Allow trailing commas in ini configuration of multiline values (Nikita Sobolev, PR 14240)
Fix false negatives involving Unions and generators or coroutines (Shantanu, PR 14224)
Fix ParamSpec constraint for types as callable (Vincent Vanlaer, PR 14153)
Fix type aliases with fixed-length tuples (Jukka Lehtosalo, PR 14184)
Fix issues with type aliases and new style unions (Jukka Lehtosalo, PR 14181)
Simplify unions less aggressively (Ivan Levkivskyi, PR 14178)
Simplify callable overlap logic (Ivan Levkivskyi, PR 14174)
Try empty context when assigning to union typed variables (Ivan Levkivskyi, PR 14151)
Improvements to recursive types (Ivan Levkivskyi, PR 14147)
Make non-numeric non-empty FORCE_COLOR truthy (Shantanu, PR 14140)
Fix to recursive type aliases (Ivan Levkivskyi, PR 14136)
Correctly handle Enum name on Python 3.11 (Ivan Levkivskyi, PR 14133)
Fix class objects falling back to metaclass for callback protocol (Ivan Levkivskyi, PR 14121)
Correctly support self types in callable ClassVar (Ivan Levkivskyi, PR 14115)
Fix type variable clash in nested positions and in attributes (Ivan Levkivskyi, PR 14095)
Allow class variable as implementation for read only attribute (Ivan Levkivskyi, PR 14081)
Prevent warnings from causing dmypy to fail (Andrzej Bartosiński, PR 14102)
Correctly process nested definitions in mypy daemon (Ivan Levkivskyi, PR 14104)
Don't consider a branch unreachable if there is a possible promotion (Ivan Levkivskyi, PR 14077)
Fix incompatible overrides of overloaded methods in concrete subclasses (Shantanu, PR 14017)
Fix new style union syntax in type aliases (Jukka Lehtosalo, PR 14008)
Fix and optimise overload compatibility checking (Shantanu, PR 14018)
Improve handling of redefinitions through imports (Shantanu, PR 13969)
Preserve (some) implicitly exported types (Shantanu, PR 13967)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alessio Izzo
Alex Waygood
Anders Kaseorg
Andrzej Bartosiński
Avasam
ChristianWitzler
Christoph Tyralla
dosisod
Harrison McCarty
Hugo van Kemenade
Hugues
Ilya Konstantinov
Ivan Levkivskyi
Jelle Zijlstra
jhance
johnthagen
Jonathan Daniel
Joshua Bronson
Jukka Lehtosalo
KotlinIsland
Lakshay Bisht
Lefteris Karapetsas
Marc Mueller
Matthew Hughes
Michael Lee
Nick Drozd
Nikita Sobolev
Richard Si
Shantanu
Stas Ilinskiy
Tim Geypens
Tin Tvrtković
Tom Schraitle
Valentin Stanciu
Vincent Vanlaer

We’d also like to thank our employer, Dropbox, for funding the mypy core team.

Posted by Stas Ilinskiy

Previous releases

For information about previous releases, refer to the posts at https://mypy-lang.blogspot.com/

Mypy Release Notes

Next Release

Mypy 1.18

Mypy Performance Improvements

Fixed‑Format Cache (Experimental)

Flexible Variable Definitions: Update

Inferred Type for Bare ClassVar

Disjoint Base Classes (@disjoint_base, PEP 800)

Miscellaneous New Mypy Features

Improvements to Match Statements

Fixes to Crashes

Experimental Free-threading Support for Mypyc

Mypyc: Support __new__

Mypyc: Faster Generators and Async Functions

Miscellaneous Mypyc Improvements

Stubtest Improvements

Documentation Updates

Other Notable Fixes and Improvements

Typeshed Updates

Acknowledgements

Mypy 1.17

Optionally Check That Match Is Exhaustive

Further Improvements to Attribute Resolution

Fixes to Nondeterministic Type Checking

Remove Support for Targeting Python 3.8

Initial Support for Python 3.14

Deprecated Flag: --force-uppercase-builtins

Mypyc: Improvements to Generators and Async Functions

Mypyc: Partial, Unsafe Support for Free Threading

Other Mypyc Fixes and Improvements

Stubgen Improvements

Miscellaneous Fixes and Improvements

Mypy 1.17.1

Acknowledgements

Mypy 1.16

Different Property Getter and Setter Types

Flexible Variable Redefinitions (Experimental)

Stricter Type Checking with Imprecise Types

Improvements to Attribute Resolution

Make Implementation for Abstract Overloads Optional

Option to Exclude Everything in .gitignore

Selectively Disable Deprecated Warnings

Annotating Native/Non-Native Classes in Mypyc

Mypyc Fixes and Improvements

Fixes to Crashes

Performance Improvements

Documentation Updates

Stubgen Improvements

Stubtest Improvements

Miscellaneous Fixes and Improvements

Acknowledgements

Mypy 1.15

Performance Improvements

Mypyc Accelerated Mypy Wheels for ARM Linux

--strict-bytes

Improvements to Reachability Analysis and Partial Type Handling in Loops

Better Discovery of Configuration Files

Better Line Numbers for Decorators and Slice Expressions

Drop Support for Python 3.8

Mypyc Improvements

Stubgen Improvements

Stubtest Improvements

Fixes to Crashes

Documentation Updates

Other Notable Fixes and Improvements

Acknowledgements

Mypy 1.14

Change to Enum Membership Semantics

Support for @deprecated Decorator (PEP 702)

Optionally Analyzing Untyped Modules

Support New Style Type Variable Defaults (PEP 696)

Improved For Loop Index Variable Type Narrowing

Mypyc Improvements

Stubgen Improvements

Stubtest Improvements

Documentation Updates

Other Notables Fixes and Improvements

Acknowledgements

Mypy 1.13

Improved Performance

Disjoint Base Classes (@disjoint_base, PEP 800)

Mypyc: Support `new`

Deprecated Flag: `--force-uppercase-builtins`

`--strict-bytes`

Support for `functools.partial`