docs/source/command_line.rst

.. _command-line:

The mypy command line
=====================

This section documents many of mypy's command line flags.  A quick
summary of command line flags can always be printed using the ``-h``
flag (or its long form ``--help``)::

  $ mypy -h
  usage: mypy [-h] [-v] [-V] [--python-version x.y] [--platform PLATFORM] [-2]
              [--ignore-missing-imports]
              [--follow-imports {normal,silent,skip,error}]
              [--disallow-untyped-calls] [--disallow-untyped-defs]
              [--check-untyped-defs] [--disallow-subclassing-any]
              [--warn-incomplete-stub] [--warn-redundant-casts]
              [--warn-no-return] [--warn-unused-ignores] [--show-error-context]
              [--fast-parser] [-i] [--cache-dir DIR] [--strict-optional]
              [--strict-optional-whitelist [GLOB [GLOB ...]]]
              [--junit-xml JUNIT_XML] [--pdb] [--show-traceback] [--stats]
              [--inferstats] [--custom-typing MODULE]
              [--custom-typeshed-dir DIR] [--scripts-are-modules]
              [--config-file CONFIG_FILE] [--show-column-numbers]
              [--find-occurrences CLASS.MEMBER] [--cobertura-xml-report DIR]
              [--html-report DIR] [--linecount-report DIR]
              [--linecoverage-report DIR] [--memory-xml-report DIR]
              [--old-html-report DIR] [--txt-report DIR] [--xml-report DIR]
              [--xslt-html-report DIR] [--xslt-txt-report DIR] [-m MODULE]
              [-c PROGRAM_TEXT] [-p PACKAGE]
              [files [files ...]]

  (etc., too long to show everything here)

Specifying files and directories to be checked
**********************************************

You've already seen ``mypy program.py`` as a way to type check the
file ``program.py``.  More generally you can pass any number of files
and directories on the command line and they will all be type checked
together.

- Files ending in ``.py`` (and stub files ending in ``.pyi``) are
  checked as Python modules.

- Files not ending in ``.py`` or ``.pyi`` are assumed to be Python
  scripts and checked as such.

- Directories representing Python packages (i.e. containing a
  ``__init__.py[i]`` file) are checked as Python packages; all
  submodules and subpackages will be checked (subpackages must
  themselves have a ``__init__.py[i]`` file).

- Directories that don't represent Python packages (i.e. not directly
  containing an ``__init__.py[i]`` file) are checked as follows:

  - All ``*.py[i]`` files contained directly therein are checked as
    toplevel Python modules;

  - All packages contained directly therein (i.e. immediate
    subdirectories with an ``__init__.py[i]`` file) are checked as
    toplevel Python packages.

One more thing about checking modules and packages: if the directory
*containing* a module or package specified on the command line has an
``__init__.py[i]`` file, mypy assigns these an absolute module name by
crawling up the path until no ``__init__.py[i]`` file is found.  For
example, suppose we run the command ``mypy foo/bar/baz.py`` where
``foo/bar/__init__.py`` exists but ``foo/__init__.py`` does not.  Then
the module name assumed is ``bar.baz`` and the directory ``foo`` is
added to mypy's module search path.  On the other hand, if
``foo/bar/__init__.py`` did not exist, ``foo/bar`` would be added to
the module search path instead, and the module name assumed is just
``baz``.

If a script (a file not ending in ``.py[i]``) is processed, the module
name assumed is always ``__main__`` (matching the behavior of the
Python interpreter).

Other ways of specifying code to be checked
*******************************************

The flag ``-m`` (long form: ``--module``) lets you specify a module
name to be found using the default module search path.  The module
name may contain dots.  For example::

  $ mypy -m html.parser

will type check the module ``html.parser`` (this happens to be a
library stub).

The flag ``-p`` (long form: ``--package``) is similar to ``-m`` but
you give it a package name and it will type check all submodules and
subpackages (recursively) of that package.  (If you pass a package
name to ``-m`` it will just type check the package's ``__init__.py``
and anything imported from there.)  For example::

  $ mypy -p html

will type check the entire ``html`` package (of library stubs).

Finally the flag ``-c`` (long form: ``--command``) will take a string
from the command line and type check it as a small program.  For
example::

  $ mypy -c 'x = [1, 2]; print(x())'

will type check that little program (and complain that ``List[int]``
is not callable).

Reading a list of files from a file
***********************************

Finally, any command-line argument starting with ``@`` reads additional
command-line arguments from the file following the ``@`` character.
This is primarily useful if you have a file containing a list of files
that you want to be type-checked: instead of using shell syntax like::

  mypy $(cat file_of_files)

you can use this instead::

  mypy @file_of_files

Such a file can also contain other flags, but a preferred way of
reading flags (not files) from a file is to use a
:ref:`configuration file <config-file>`.


.. _finding-imports:

How imports are found
*********************

When mypy encounters an `import` statement it tries to find the module
on the file system, similar to the way Python finds it.
However, there are some differences.

First, mypy has its own search path.
This is computed from the following items:

- The ``MYPYPATH`` environment variable
  (a colon-separated list of directories).
- The directories containing the sources given on the command line
  (see below).
- The relevant directories of the
  `typeshed <https://github.com/python/typeshed>`_ repo.

For sources given on the command line, the path is adjusted by crawling
up from the given file or package to the nearest directory that does not
contain an ``__init__.py`` or ``__init__.pyi`` file.

Second, mypy searches for stub files in addition to regular Python files
and packages.
The rules for searching a module ``foo`` are as follows:

- The search looks in each of the directories in the search path
  (see above) until a match is found.
- If a package named ``foo`` is found (i.e. a directory
  ``foo`` containing an ``__init__.py`` or ``__init__.pyi`` file)
  that's a match.
- If a stub file named ``foo.pyi`` is found, that's a match.
- If a Python module named ``foo.py`` is found, that's a match.

These matches are tried in order, so that if multiple matches are found
in the same directory on the search path
(e.g. a package and a Python file, or a stub file and a Python file)
the first one in the above list wins.

In particular, if a Python file and a stub file are both present in the
same directory on the search path, only the stub file is used.
(However, if the files are in different directories, the one found
in the earlier directory is used.)

NOTE: These rules are relevant to the following section too:
the ``--follow-imports`` flag described below is applied _after_ the
above algorithm has determined which package, stub or module to use.

.. _follow-imports:

Following imports or not?
*************************

When you're first attacking a large existing codebase with mypy, you
may only want to check selected files.  For example, you may only want
to check those files to which you have already added annotations.
This is easily accomplished using a shell pipeline like this::

  mypy $(find . -name \*.py | xargs grep -l '# type:')

(While there are many improvements possible to make this example more
robust, this is not the place for a tutorial in shell programming.)

However, by default mypy doggedly tries to :ref:`follow imports
<finding-imports>`.  This may cause several types of problems that you
may want to silence during your initial conquest:

- Your code may import library modules for which no stub files exist
  yet.  This can cause a lot of errors like the following::

    main.py:1: error: No library stub file for standard library module 'antigravity'
    main.py:2: error: No library stub file for module 'flask'
    main.py:3: error: Cannot find module named 'sir_not_appearing_in_this_film'

  If you see only a few of these you may be able to silence them by
  putting ``# type: ignore`` on the respective ``import`` statements,
  but it's usually easier to silence all such errors by using
  :ref:`--ignore-missing-imports <ignore-missing-imports>`.

- Your project's directory structure may hinder mypy in finding
  certain modules that are part of your project, e.g. modules hidden
  away in a subdirectory that's not a package.  You can usually deal
  with this by setting the ``MYPYPATH`` variable (see
  :ref:`finding-imports`).

- When following imports mypy may find a module that's part of your
  project but which you haven't annotated yet, mypy may report errors
  for the top level code in that module (where the top level includes
  class bodies and function/method default values).  Here the
  ``--follow-imports`` flag comes in handy.

The ``--follow-imports`` flag takes a mandatory string value that can
take one of four values.  It only applies to modules for which a
``.py`` file is found (but no corresponding ``.pyi`` stub file) and
that are not given on the command line.  Passing a package or
directory on the command line implies all modules in that package or
directory.  The four possible values are:

- ``normal`` (the default) follow imports normally and type check all
  top level code (as well as the bodies of all functions and methods
  with at least one type annotation in the signature).

- ``silent`` follow imports normally and even "type check" them
  normally, but *suppress any error messages*. This is typically the
  best option for a new codebase.

- ``skip`` *don't* follow imports, silently replacing the module (and
  everything imported *from* it) with an object of type ``Any``.
  (This option used to be known as ``--silent-imports`` and while it
  is very powerful it can also cause hard-to-debug errors, hence the
  recommendation of using ``silent`` instead.)

- ``error`` the same behavior as ``skip`` but not quite as silent --
  it flags the import as an error, like this::

    main.py:1: note: Import of 'submodule' ignored
    main.py:1: note: (Using --follow-imports=error, module not passed on command line)


Additional command line flags
*****************************

Here are some more useful flags:

.. _ignore-missing-imports:

- ``--ignore-missing-imports`` suppresses error messages about imports
  that cannot be resolved (see :ref:`follow-imports` for some examples).

- ``--strict-optional`` enables experimental strict checking of ``Optional[...]``
  types and ``None`` values. Without this option, mypy doesn't generally check the
  use of ``None`` values -- they are valid everywhere. See :ref:`strict_optional` for
  more about this feature.

- ``--strict-optional-whitelist`` attempts to suppress strict Optional-related
  errors in non-whitelisted files.  Takes an arbitrary number of globs as the
  whitelist.  This option is intended to be used to incrementally roll out
  ``--strict-optional`` to a large codebase that already has mypy annotations.
  However, this flag comes with some significant caveats.  It does not suppress
  all errors caused by turning on ``--strict-optional``, only most of them, so
  there may still be a bit of upfront work to be done before it can be used in
  CI.  It will also suppress some errors that would be caught in a
  non-strict-Optional run.  Therefore, when using this flag, you should also
  re-check your code without ``--strict-optional`` to ensure new type errors
  are not introduced.

- ``--disallow-untyped-defs`` reports an error whenever it encounters
  a function definition without type annotations.

- ``--check-untyped-defs`` is less severe than the previous option --
  it type checks the body of every function, regardless of whether it
  has type annotations.  (By default the bodies of functions without
  annotations are not type checked.)  It will assume all arguments
  have type ``Any`` and always infer ``Any`` as the return type.

- ``--disallow-untyped-calls`` reports an error whenever a function
  with type annotations calls a function defined without annotations.

.. _disallow-subclassing-any:

- ``--disallow-subclassing-any`` reports an error whenever a class
  subclasses a value of type ``Any``.  This may occur when the base
  class is imported from a module that doesn't exist (when using
  :ref:`--ignore-missing-imports <ignore-missing-imports>`) or is
  ignored due to :ref:`--follow-imports=skip <follow-imports>` or a
  ``# type: ignore`` comment on the ``import`` statement.  Since the
  module is silenced, the imported class is given a type of ``Any``.
  By default mypy will assume that the subclass correctly inherited
  the base class even though that may not actually be the case.  This
  flag makes mypy raise an error instead.

- ``--incremental`` is an experimental option that enables incremental
  type checking. When enabled, mypy caches results from previous runs
  to speed up type checking. Incremental mode can help when most parts
  of your program haven't changed since the previous mypy run.

- ``--fast-parser`` enables an experimental parser implemented in C that
  is faster than the default parser and supports multi-line comment
  function annotations (see :ref:`multi_line_annotation` for the details).

- ``--python-version X.Y`` will make mypy typecheck your code as if it were
  run under Python version X.Y. Without this option, mypy will default to using
  whatever version of Python is running mypy. Note that the ``-2`` and
  ``--py2`` flags are aliases for ``--python-version 2.7``. See
  :ref:`version_and_platform_checks` for more about this feature.

- ``--platform PLATFORM`` will make mypy typecheck your code as if it were
  run under the the given operating system. Without this option, mypy will
  default to using whatever operating system you are currently using. See
  :ref:`version_and_platform_checks` for more about this feature.

- ``--show-column-numbers`` will add column offsets to error messages,
  for example, the following indicates an error in line 12, column 9
  (note that column offsets are 0-based):

  .. code-block:: python

     main.py:12:9: error: Unsupported operand types for / ("int" and "str")

- ``--scripts-are-modules`` will give command line arguments that
  appear to be scripts (i.e. files whose name does not end in ``.py``)
  a module name derived from the script name rather than the fixed
  name ``__main__``.  This allows checking more than one script in a
  single mypy invocation.  (The default ``__main__`` is technically
  more correct, but if you have many scripts that import a large
  package, the behavior enabled by this flag is often more
  convenient.)

- ``--custom-typeshed-dir DIR`` specifies the directory where mypy looks for
  typeshed stubs, instead of the typeshed that ships with mypy.  This is
  primarily intended to make it easier to test typeshed changes before
  submitting them upstream, but also allows you to use a forked version of
  typeshed.

.. _config-file-flag:

- ``--config-file CONFIG_FILE`` causes configuration settings to be
  read from the given file.  By default settings are read from ``mypy.ini``
  in the current directory.  Settings override mypy's built-in defaults
  and command line flags can override settings.  See :ref:`config-file`
  for the syntax of configuration files.

- ``--junit-xml JUNIT_XML`` will make mypy generate a JUnit XML test
  result document with type checking results. This can make it easier
  to integrate mypy with continuous integration (CI) tools.

- ``--find-occurrences CLASS.MEMBER`` will make mypy print out all
  usages of a class member based on static type information. This
  feature is experimental.

- ``--cobertura-xml-report DIR`` causes mypy to generate a Cobertura
  XML type checking coverage report.

- ``--warn-no-return`` causes mypy to generate errors for missing return
  statements on some execution paths. Mypy doesn't generate these errors
  for functions with ``None`` or ``Any`` return types. Mypy
  also currently ignores functions with an empty body or a body that is
  just ellipsis (``...``), since these can be valid as abstract methods.

For the remaining flags you can read the full ``mypy -h`` output.

.. note::

   Command line flags are liable to change between releases.

.. _integrating-mypy:

Integrating mypy into another Python application
************************************************

It is possible to integrate mypy into another Python 3 application by
importing ``mypy.api`` and calling the ``run`` function with exactly the string
you would have passed to mypy from the command line.

Function ``run`` returns a tuple of strings:
``(<normal_report>, <error_report>)``, in which ``<normal_report>`` is what mypy
normally writes to ``sys.stdout`` and ``<error_report>`` is what mypy normally
writes to ``sys.stderr``.

A trivial example of this is the following::

    import sys
    from mypy import api

    result = api.run(' '.join(sys.argv[1:]))

    if result[0]:
        print('\nType checking report:\n')
        print(result[0])  # stdout

    if result[1]:
        print('\nError report:\n')
        print(result[1])  # stderr