README.md - third_party/github.com/python/mypy - Git at Google

 <img src="http://mypy-lang.org/static/mypy_light.svg" alt="mypy logo" width="300px"/>

 Mypy: Optional Static Typing for Python
 =======================================

 [![Build Status](https://api.travis-ci.org/python/mypy.svg?branch=master)](https://travis-ci.org/python/mypy)
 [![Chat at https://gitter.im/python/typing](https://badges.gitter.im/python/typing.svg)](https://gitter.im/python/typing?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)


 Got a question? Join us on Gitter!
 ----------------------------------

 We don't have a mailing list; but we are always happy to answer
 questions on [gitter chat](https://gitter.im/python/typing).  If you are
 sure you've found a bug please search our issue trackers for a
 duplicate before filing a new issue:

 - [mypy tracker](https://github.com/python/mypy/issues)
   for mypy issues
 - [typeshed tracker](https://github.com/python/typeshed/issues)
   for issues with specific modules
 - [typing tracker](https://github.com/python/typing/issues)
   for discussion of new type system features (PEP 484 changes) and
   runtime bugs in the typing module

 What is mypy?
 -------------

 Mypy is an optional static type checker for Python.  You can add type
 hints ([PEP 484](https://www.python.org/dev/peps/pep-0484/)) to your
 Python programs, and use mypy to type check them statically.
 Find bugs in your programs without even running them!

 You can mix dynamic and static typing in your programs. You can always
 fall back to dynamic typing when static typing is not convenient, such
 as for legacy code.

 Here is a small example to whet your appetite (Python 3):

 ```python
 from typing import Iterator

 def fib(n: int) -> Iterator[int]:
     a, b = 0, 1
     while a < n:
         yield a
         a, b = b, a + b
 ```
 See [the documentation](http://mypy.readthedocs.io/en/stable/introduction.html) for more examples.

 For Python 2.7, the standard annotations are written as comments:
 ```python
 def is_palindrome(s):
     # type: (str) -> bool
     return s == s[::-1]
 ```

 See [the documentation for Python 2 support](http://mypy.readthedocs.io/en/latest/python2.html).

 Mypy is in development; some features are missing and there are bugs.
 See 'Development status' below.

 Requirements
 ------------

 You need Python 3.5 or later to run mypy.  You can have multiple Python
 versions (2.x and 3.x) installed on the same system without problems.

 In Ubuntu, Mint and Debian you can install Python 3 like this:

     $ sudo apt-get install python3 python3-pip

 For other Linux flavors, macOS and Windows, packages are available at

   http://www.python.org/getit/


 Quick start
 -----------

 Mypy can be installed using pip:

     $ python3 -m pip install -U mypy

 If you want to run the latest version of the code, you can install from git:

     $ python3 -m pip install -U git+git://github.com/python/mypy.git


 Now, if Python on your system is configured properly (else see
 "Troubleshooting" below), you can type-check the [statically typed parts] of a
 program like this:

     $ mypy PROGRAM

 You can always use a Python interpreter to run your statically typed
 programs, even if they have type errors:

     $ python3 PROGRAM

 You can also try mypy in an [online playground](https://mypy-play.net/) (developed by
 Yusuke Miyazaki).

 [statically typed parts]: https://mypy.readthedocs.io/en/latest/getting_started.html#function-signatures-and-dynamic-vs-static-typing


 IDE, Linter Integrations, and Pre-commit
 ----------------------------------------

 Mypy can be integrated into popular IDEs:

 * Vim:
   * Using [Syntastic](https://github.com/vim-syntastic/syntastic): in `~/.vimrc` add
     `let g:syntastic_python_checkers=['mypy']`
   * Using [ALE](https://github.com/dense-analysis/ale): should be enabled by default when `mypy` is installed,
     or can be explicitly enabled by adding `let b:ale_linters = ['mypy']` in `~/vim/ftplugin/python.vim`
 * Emacs: using [Flycheck](https://github.com/flycheck/) and [Flycheck-mypy](https://github.com/lbolla/emacs-flycheck-mypy)
 * Sublime Text: [SublimeLinter-contrib-mypy](https://github.com/fredcallaway/SublimeLinter-contrib-mypy)
 * Atom: [linter-mypy](https://atom.io/packages/linter-mypy)
 * PyCharm: [mypy plugin](https://github.com/dropbox/mypy-PyCharm-plugin) (PyCharm integrates
   [its own implementation of PEP 484](https://www.jetbrains.com/help/pycharm/type-hinting-in-product.html))
 * VS Code: provides [basic integration](https://code.visualstudio.com/docs/python/linting#_mypy) with mypy.

 Mypy can also be integrated into [Flake8] using [flake8-mypy], or
 can be set up as a pre-commit hook using [pre-commit mirrors-mypy].

 [Flake8]: http://flake8.pycqa.org/
 [flake8-mypy]: https://github.com/ambv/flake8-mypy
 [pre-commit mirrors-mypy]: https://github.com/pre-commit/mirrors-mypy

 Web site and documentation
 --------------------------

 Documentation and additional information is available at the web site:

   http://www.mypy-lang.org/

 Or you can jump straight to the documentation:

   http://mypy.readthedocs.io/


 Troubleshooting
 ---------------

 Depending on your configuration, you may have to run `pip` like
 this:

     $ python3 -m pip install -U mypy

 This should automatically install the appropriate version of
 mypy's parser, typed-ast.  If for some reason it does not, you
 can install it manually:

     $ python3 -m pip install -U typed-ast

 If the `mypy` command isn't found after installation: After
 `python3 -m pip install`, the `mypy` script and
 dependencies, including the `typing` module, will be installed to
 system-dependent locations.  Sometimes the script directory will not
 be in `PATH`, and you have to add the target directory to `PATH`
 manually or create a symbolic link to the script.  In particular, on
 macOS, the script may be installed under `/Library/Frameworks`:

     /Library/Frameworks/Python.framework/Versions/<version>/bin

 In Windows, the script is generally installed in
 `\PythonNN\Scripts`. So, type check a program like this (replace
 `\Python34` with your Python installation path):

     C:\>\Python34\python \Python34\Scripts\mypy PROGRAM

 ### Working with `virtualenv`

 If you are using [`virtualenv`](https://virtualenv.pypa.io/en/stable/),
 make sure you are running a python3 environment. Installing via `pip3`
 in a v2 environment will not configure the environment to run installed
 modules from the command line.

     $ python3 -m pip install -U virtualenv
     $ python3 -m virtualenv env


 Quick start for contributing to mypy
 ------------------------------------

 If you want to contribute, first clone the mypy git repository:

     $ git clone --recurse-submodules https://github.com/python/mypy.git

 If you've already cloned the repo without `--recurse-submodules`,
 you need to pull in the typeshed repo as follows:

     $ git submodule init
     $ git submodule update

 Either way you should now have a subdirectory `typeshed` inside your mypy repo,
 your folders tree should be like `mypy/mypy/typeshed`, containing a
 clone of the typeshed repo (`https://github.com/python/typeshed`).

 From the mypy directory, use pip to install mypy:

     $ cd mypy
     $ python3 -m pip install -U .

 Replace `python3` with your Python 3 interpreter.  You may have to do
 the above as root. For example, in Ubuntu:

     $ sudo python3 -m pip install -U .

 Now you can use the `mypy` program just as above.  In case of trouble
 see "Troubleshooting" above.


 Working with the git version of mypy
 ------------------------------------

 mypy contains a submodule, "typeshed". See http://github.com/python/typeshed.
 This submodule contains types for the Python standard library.

 Due to the way git submodules work, you'll have to do
 ```
   git submodule update typeshed
 ```
 whenever you change branches, merge, rebase, or pull.

 (It's possible to automate this: Search Google for "git hook update submodule")


 Tests
 -----

 The basic way to run tests:

     $ pip3 install -r test-requirements.txt
     $ python2 -m pip install -U typing
     $ ./runtests.py

 For more on the tests, see [Test README.md](test-data/unit/README.md)


 Development status
 ------------------

 Mypy is beta software, but it has already been used in production
 for several years at Dropbox, and it has an extensive test suite.

 See [the roadmap](ROADMAP.md) if you are interested in plans for the
 future.


 Changelog
 ---------

 Follow mypy's updates on the blog: http://mypy-lang.blogspot.com/


 Issue tracker
 -------------

 Please report any bugs and enhancement ideas using the mypy issue
 tracker: https://github.com/python/mypy/issues

 If you have any questions about using mypy or types, please ask
 in the typing gitter instead: https://gitter.im/python/typing


 Compiled version of mypy
 ------------------------

 We have built a compiled version of mypy using the [mypyc
 compiler](https://github.com/python/mypy/tree/master/mypyc) for
 mypy-annotated Python code. It is approximately 4 times faster than
 interpreted mypy and is available (and the default) for 64-bit
 Windows, macOS, and Linux.

 To install an interpreted mypy instead, use:

     $ python3 -m pip install --no-binary mypy -U mypy

 If you wish to test out the compiled version of a development
 version of mypy, you can directly install a binary from
 https://github.com/mypyc/mypy_mypyc-wheels/releases/latest.


 Help wanted
 -----------

 Any help in testing, development, documentation and other tasks is
 highly appreciated and useful to the project. There are tasks for
 contributors of all experience levels. If you're just getting started,
 ask on the [gitter chat](https://gitter.im/python/typing) for ideas of good
 beginner issues.

 For more details, see the file [CONTRIBUTING.md](CONTRIBUTING.md).


 License
 -------

 Mypy is licensed under the terms of the MIT License (see the file
 LICENSE).
	<img src="http://mypy-lang.org/static/mypy_light.svg" alt="mypy logo" width="300px"/>

	Mypy: Optional Static Typing for Python
	=======================================

	[![Build Status](https://api.travis-ci.org/python/mypy.svg?branch=master)](https://travis-ci.org/python/mypy)
	[![Chat at https://gitter.im/python/typing](https://badges.gitter.im/python/typing.svg)](https://gitter.im/python/typing?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
	[![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)


	Got a question? Join us on Gitter!
	----------------------------------

	We don't have a mailing list; but we are always happy to answer
	questions on [gitter chat](https://gitter.im/python/typing). If you are
	sure you've found a bug please search our issue trackers for a
	duplicate before filing a new issue:

	- [mypy tracker](https://github.com/python/mypy/issues)
	for mypy issues
	- [typeshed tracker](https://github.com/python/typeshed/issues)
	for issues with specific modules
	- [typing tracker](https://github.com/python/typing/issues)
	for discussion of new type system features (PEP 484 changes) and
	runtime bugs in the typing module

	What is mypy?
	-------------

	Mypy is an optional static type checker for Python. You can add type
	hints ([PEP 484](https://www.python.org/dev/peps/pep-0484/)) to your
	Python programs, and use mypy to type check them statically.
	Find bugs in your programs without even running them!

	You can mix dynamic and static typing in your programs. You can always
	fall back to dynamic typing when static typing is not convenient, such
	as for legacy code.

	Here is a small example to whet your appetite (Python 3):

	```python
	from typing import Iterator

	def fib(n: int) -> Iterator[int]:
	a, b = 0, 1
	while a < n:
	yield a
	a, b = b, a + b
	```
	See [the documentation](http://mypy.readthedocs.io/en/stable/introduction.html) for more examples.

	For Python 2.7, the standard annotations are written as comments:
	```python
	def is_palindrome(s):
	# type: (str) -> bool
	return s == s[::-1]
	```

	See [the documentation for Python 2 support](http://mypy.readthedocs.io/en/latest/python2.html).

	Mypy is in development; some features are missing and there are bugs.
	See 'Development status' below.

	Requirements
	------------

	You need Python 3.5 or later to run mypy. You can have multiple Python
	versions (2.x and 3.x) installed on the same system without problems.

	In Ubuntu, Mint and Debian you can install Python 3 like this:

	$ sudo apt-get install python3 python3-pip

	For other Linux flavors, macOS and Windows, packages are available at

	http://www.python.org/getit/


	Quick start
	-----------

	Mypy can be installed using pip:

	$ python3 -m pip install -U mypy

	If you want to run the latest version of the code, you can install from git:

	$ python3 -m pip install -U git+git://github.com/python/mypy.git


	Now, if Python on your system is configured properly (else see
	"Troubleshooting" below), you can type-check the [statically typed parts] of a
	program like this:

	$ mypy PROGRAM

	You can always use a Python interpreter to run your statically typed
	programs, even if they have type errors:

	$ python3 PROGRAM

	You can also try mypy in an [online playground](https://mypy-play.net/) (developed by
	Yusuke Miyazaki).

	[statically typed parts]: https://mypy.readthedocs.io/en/latest/getting_started.html#function-signatures-and-dynamic-vs-static-typing


	IDE, Linter Integrations, and Pre-commit
	----------------------------------------

	Mypy can be integrated into popular IDEs:

	* Vim:
	* Using [Syntastic](https://github.com/vim-syntastic/syntastic): in `~/.vimrc` add
	`let g:syntastic_python_checkers=['mypy']`
	* Using [ALE](https://github.com/dense-analysis/ale): should be enabled by default when `mypy` is installed,
	or can be explicitly enabled by adding `let b:ale_linters = ['mypy']` in `~/vim/ftplugin/python.vim`
	* Emacs: using [Flycheck](https://github.com/flycheck/) and [Flycheck-mypy](https://github.com/lbolla/emacs-flycheck-mypy)
	* Sublime Text: [SublimeLinter-contrib-mypy](https://github.com/fredcallaway/SublimeLinter-contrib-mypy)
	* Atom: [linter-mypy](https://atom.io/packages/linter-mypy)
	* PyCharm: [mypy plugin](https://github.com/dropbox/mypy-PyCharm-plugin) (PyCharm integrates
	[its own implementation of PEP 484](https://www.jetbrains.com/help/pycharm/type-hinting-in-product.html))
	* VS Code: provides [basic integration](https://code.visualstudio.com/docs/python/linting#_mypy) with mypy.

	Mypy can also be integrated into [Flake8] using [flake8-mypy], or
	can be set up as a pre-commit hook using [pre-commit mirrors-mypy].

	[Flake8]: http://flake8.pycqa.org/
	[flake8-mypy]: https://github.com/ambv/flake8-mypy
	[pre-commit mirrors-mypy]: https://github.com/pre-commit/mirrors-mypy

	Web site and documentation
	--------------------------

	Documentation and additional information is available at the web site:

	http://www.mypy-lang.org/

	Or you can jump straight to the documentation:

	http://mypy.readthedocs.io/


	Troubleshooting
	---------------

	Depending on your configuration, you may have to run `pip` like
	this:

	$ python3 -m pip install -U mypy

	This should automatically install the appropriate version of
	mypy's parser, typed-ast. If for some reason it does not, you
	can install it manually:

	$ python3 -m pip install -U typed-ast

	If the `mypy` command isn't found after installation: After
	`python3 -m pip install`, the `mypy` script and
	dependencies, including the `typing` module, will be installed to
	system-dependent locations. Sometimes the script directory will not
	be in `PATH`, and you have to add the target directory to `PATH`
	manually or create a symbolic link to the script. In particular, on
	macOS, the script may be installed under `/Library/Frameworks`:

	/Library/Frameworks/Python.framework/Versions/<version>/bin

	In Windows, the script is generally installed in
	`\PythonNN\Scripts`. So, type check a program like this (replace
	`\Python34` with your Python installation path):

	C:\>\Python34\python \Python34\Scripts\mypy PROGRAM

	### Working with `virtualenv`

	If you are using [`virtualenv`](https://virtualenv.pypa.io/en/stable/),
	make sure you are running a python3 environment. Installing via `pip3`
	in a v2 environment will not configure the environment to run installed
	modules from the command line.

	$ python3 -m pip install -U virtualenv
	$ python3 -m virtualenv env


	Quick start for contributing to mypy
	------------------------------------

	If you want to contribute, first clone the mypy git repository:

	$ git clone --recurse-submodules https://github.com/python/mypy.git

	If you've already cloned the repo without `--recurse-submodules`,
	you need to pull in the typeshed repo as follows:

	$ git submodule init
	$ git submodule update

	Either way you should now have a subdirectory `typeshed` inside your mypy repo,
	your folders tree should be like `mypy/mypy/typeshed`, containing a
	clone of the typeshed repo (`https://github.com/python/typeshed`).

	From the mypy directory, use pip to install mypy:

	$ cd mypy
	$ python3 -m pip install -U .

	Replace `python3` with your Python 3 interpreter. You may have to do
	the above as root. For example, in Ubuntu:

	$ sudo python3 -m pip install -U .

	Now you can use the `mypy` program just as above. In case of trouble
	see "Troubleshooting" above.


	Working with the git version of mypy
	------------------------------------

	mypy contains a submodule, "typeshed". See http://github.com/python/typeshed.
	This submodule contains types for the Python standard library.

	Due to the way git submodules work, you'll have to do
	```
	git submodule update typeshed
	```
	whenever you change branches, merge, rebase, or pull.

	(It's possible to automate this: Search Google for "git hook update submodule")


	Tests
	-----

	The basic way to run tests:

	$ pip3 install -r test-requirements.txt
	$ python2 -m pip install -U typing
	$ ./runtests.py

	For more on the tests, see [Test README.md](test-data/unit/README.md)


	Development status
	------------------

	Mypy is beta software, but it has already been used in production
	for several years at Dropbox, and it has an extensive test suite.

	See [the roadmap](ROADMAP.md) if you are interested in plans for the
	future.


	Changelog
	---------

	Follow mypy's updates on the blog: http://mypy-lang.blogspot.com/


	Issue tracker
	-------------

	Please report any bugs and enhancement ideas using the mypy issue
	tracker: https://github.com/python/mypy/issues

	If you have any questions about using mypy or types, please ask
	in the typing gitter instead: https://gitter.im/python/typing


	Compiled version of mypy
	------------------------

	We have built a compiled version of mypy using the [mypyc
	compiler](https://github.com/python/mypy/tree/master/mypyc) for
	mypy-annotated Python code. It is approximately 4 times faster than
	interpreted mypy and is available (and the default) for 64-bit
	Windows, macOS, and Linux.

	To install an interpreted mypy instead, use:

	$ python3 -m pip install --no-binary mypy -U mypy

	If you wish to test out the compiled version of a development
	version of mypy, you can directly install a binary from
	https://github.com/mypyc/mypy_mypyc-wheels/releases/latest.


	Help wanted
	-----------

	Any help in testing, development, documentation and other tasks is
	highly appreciated and useful to the project. There are tasks for
	contributors of all experience levels. If you're just getting started,
	ask on the [gitter chat](https://gitter.im/python/typing) for ideas of good
	beginner issues.

	For more details, see the file [CONTRIBUTING.md](CONTRIBUTING.md).


	License
	-------

	Mypy is licensed under the terms of the MIT License (see the file
	LICENSE).