.. _runtime_troubles:

Annotation issues at runtime
============================

Idiomatic use of type annotations can sometimes run up against what a given
version of Python considers legal code. This section describes these scenarios
and explains how to get your code running again. Generally speaking, we have
three tools at our disposal:

* For Python 3.7 through 3.9, use of ``from __future__ import annotations``
  (:pep:`563`), made the default in Python 3.10 and later
* Use of string literal types or type comments
* Use of ``typing.TYPE_CHECKING``

We provide a description of these before moving onto discussion of specific
problems you may encounter.

.. _string-literal-types:

String literal types
--------------------

Type comments can't cause runtime errors because comments are not evaluated by
Python. In a similar way, using string literal types sidesteps the problem of
annotations that would cause runtime errors.

Any type can be entered as a string literal, and you can combine
string-literal types with non-string-literal types freely:

.. code-block:: python

   def f(a: List['A']) -> None: ...  # OK
   def g(n: 'int') -> None: ...      # OK, though not useful

   class A: pass

String literal types are never needed in ``# type:`` comments and :ref:`stub files <stub-files>`.

String literal types must be defined (or imported) later *in the same module*.
They cannot be used to leave cross-module references unresolved.  (For dealing
with import cycles, see :ref:`import-cycles`.)

.. _future-annotations:

Future annotations import (PEP 563)
-----------------------------------

Many of the issues described here are caused by Python trying to evaluate
annotations. From Python 3.10 on, Python will no longer attempt to evaluate
function and variable annotations. This behaviour is made available in Python
3.7 and later through the use of ``from __future__ import annotations``.

This can be thought of as automatic string literal-ification of all function and
variable annotations. Note that function and variable annotations are still
required to be valid Python syntax. For more details, see :pep:`563`.

.. note::

    Even with the ``__future__`` import, there are some scenarios that could
    still require string literals or result in errors, typically involving use
    of forward references or generics in:

    * :ref:`type aliases <type-aliases>`;
    * :ref:`casts <casts>`;
    * type definitions (see :py:class:`~typing.TypeVar`, :py:func:`~typing.NewType`, :py:class:`~typing.NamedTuple`);
    * base classes.

    .. code-block:: python

        # base class example
        from __future__ import annotations
        class A(Tuple['B', 'C']): ... # String literal types needed here
        class B: ...
        class C: ...

.. note::

    Some libraries may have use cases for dynamic evaluation of annotations, for
    instance, through use of ``typing.get_type_hints`` or ``eval``. If your
    annotation would raise an error when evaluated (say by using :pep:`604`
    syntax with Python 3.9), you may need to be careful when using such
    libraries.

.. _typing-type-checking:

typing.TYPE_CHECKING
--------------------

The :py:mod:`typing` module defines a :py:data:`~typing.TYPE_CHECKING` constant
that is ``False`` at runtime but treated as ``True`` while type checking.

Since code inside ``if TYPE_CHECKING:`` is not executed at runtime, it provides
a convenient way to tell mypy something without the code being evaluated at
runtime. This is most useful for resolving :ref:`import cycles <import-cycles>`.

.. note::

   Python 3.5.1 and below don't have :py:data:`~typing.TYPE_CHECKING`. An
   alternative is to define a constant named ``MYPY`` that has the value
   ``False`` at runtime. Mypy considers it to be ``True`` when type checking.

Class name forward references
-----------------------------

Python does not allow references to a class object before the class is
defined (aka forward reference). Thus this code does not work as expected:

.. code-block:: python

   def f(x: A) -> None: ...  # NameError: name 'A' is not defined
   class A: ...

Starting from Python 3.7, you can add ``from __future__ import annotations`` to
resolve this, as discussed earlier:

.. code-block:: python

   from __future__ import annotations

   def f(x: A) -> None: ...  # OK
   class A: ...

For Python 3.6 and below, you can enter the type as a string literal or type comment:

.. code-block:: python

   def f(x: 'A') -> None: ...  # OK

   # Also OK
   def g(x):  # type: (A) -> None
       ...

   class A: ...

Of course, instead of using future annotations import or string literal types,
you could move the function definition after the class definition. This is not
always desirable or even possible, though.

.. _import-cycles:

Import cycles
-------------

An import cycle occurs where module A imports module B and module B
imports module A (perhaps indirectly, e.g. ``A -> B -> C -> A``).
Sometimes in order to add type annotations you have to add extra
imports to a module and those imports cause cycles that didn't exist
before. This can lead to errors at runtime like:

.. code-block::

   ImportError: cannot import name 'b' from partially initialized module 'A' (most likely due to a circular import)

If those cycles do become a problem when running your program, there's a trick:
if the import is only needed for type annotations and you're using a) the
:ref:`future annotations import<future-annotations>`, or b) string literals or type
comments for the relevant annotations, you can write the imports inside ``if
TYPE_CHECKING:`` so that they are not executed at runtime. Example:

File ``foo.py``:

.. code-block:: python

   from typing import List, TYPE_CHECKING

   if TYPE_CHECKING:
       import bar

   def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]':
       return [arg]

File ``bar.py``:

.. code-block:: python

   from typing import List
   from foo import listify

   class BarClass:
       def listifyme(self) -> 'List[BarClass]':
           return listify(self)

.. _not-generic-runtime:

Using classes that are generic in stubs but not at runtime
----------------------------------------------------------

Some classes are declared as :ref:`generic<generic-classes>` in stubs, but not
at runtime.

In Python 3.8 and earlier, there are several examples within the standard library,
for instance, :py:class:`os.PathLike` and :py:class:`queue.Queue`. Subscripting
such a class will result in a runtime error:

.. code-block:: python

   from queue import Queue

   class Tasks(Queue[str]):  # TypeError: 'type' object is not subscriptable
       ...

   results: Queue[int] = Queue()  # TypeError: 'type' object is not subscriptable

To avoid errors from use of these generics in annotations, just use the
:ref:`future annotations import<future-annotations>` (or string literals or type
comments for Python 3.6 and below).

To avoid errors when inheriting from these classes, things are a little more
complicated and you need to use :ref:`typing.TYPE_CHECKING
<typing-type-checking>`:

.. code-block:: python

   from typing import TYPE_CHECKING
   from queue import Queue

   if TYPE_CHECKING:
       BaseQueue = Queue[str]  # this is only processed by mypy
   else:
       BaseQueue = Queue  # this is not seen by mypy but will be executed at runtime

   class Tasks(BaseQueue):  # OK
       ...

   task_queue: Tasks
   reveal_type(task_queue.get())  # Reveals str

If your subclass is also generic, you can use the following:

.. code-block:: python

   from typing import TYPE_CHECKING, TypeVar, Generic
   from queue import Queue

   _T = TypeVar("_T")
   if TYPE_CHECKING:
       class _MyQueueBase(Queue[_T]): pass
   else:
       class _MyQueueBase(Generic[_T], Queue): pass

   class MyQueue(_MyQueueBase[_T]): pass

   task_queue: MyQueue[str]
   reveal_type(task_queue.get())  # Reveals str

In Python 3.9, we can just inherit directly from ``Queue[str]`` or ``Queue[T]``
since its :py:class:`queue.Queue` implements :py:meth:`__class_getitem__`, so
the class object can be subscripted at runtime without issue.

Using types defined in stubs but not at runtime
-----------------------------------------------

Sometimes stubs that you're using may define types you wish to re-use that do
not exist at runtime. Importing these types naively will cause your code to fail
at runtime with ``ImportError`` or ``ModuleNotFoundError``. Similar to previous
sections, these can be dealt with by using :ref:`typing.TYPE_CHECKING
<typing-type-checking>`:

.. code-block:: python

   from typing import TYPE_CHECKING
   if TYPE_CHECKING:
       from _typeshed import SupportsLessThan

.. _generic-builtins:

Using generic builtins
----------------------

Starting with Python 3.9 (:pep:`585`), the type objects of many collections in
the standard library support subscription at runtime. This means that you no
longer have to import the equivalents from :py:mod:`typing`; you can simply use
the built-in collections or those from :py:mod:`collections.abc`:

.. code-block:: python

   from collections.abc import Sequence
   x: list[str]
   y: dict[int, str]
   z: Sequence[str] = x

There is limited support for using this syntax in Python 3.7 and later as well.
If you use ``from __future__ import annotations``, mypy will understand this
syntax in annotations. However, since this will not be supported by the Python
interpreter at runtime, make sure you're aware of the caveats mentioned in the
notes at :ref:`future annotations import<future-annotations>`.

Using X | Y syntax for Unions
-----------------------------

Starting with Python 3.10 (:pep:`604`), you can spell union types as ``x: int |
str``, instead of ``x: typing.Union[int, str]``.

There is limited support for using this syntax in Python 3.7 and later as well.
If you use ``from __future__ import annotations``, mypy will understand this
syntax in annotations, string literal types, type comments and stub files.
However, since this will not be supported by the Python interpreter at runtime
(if evaluated, ``int | str`` will raise ``TypeError: unsupported operand type(s)
for |: 'type' and 'type'``), make sure you're aware of the caveats mentioned in
the notes at :ref:`future annotations import<future-annotations>`.

Using new additions to the typing module
----------------------------------------

You may find yourself wanting to use features added to the :py:mod:`typing`
module in earlier versions of Python than the addition, for example, using any
of ``Literal``, ``Protocol``, ``TypedDict`` with Python 3.6.

The easiest way to do this is to install and use the ``typing_extensions``
package from PyPI for the relevant imports, for example:

.. code-block:: python

   from typing_extensions import Literal
   x: Literal["open", "close"]

If you don't want to rely on ``typing_extensions`` being installed on newer
Pythons, you could alternatively use:

.. code-block:: python

   import sys
   if sys.version_info >= (3, 8):
       from typing import Literal
   else:
       from typing_extensions import Literal

   x: Literal["open", "close"]

This plays nicely well with following :pep:`508` dependency specification:
``typing_extensions; python_version<"3.8"``