README.md

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

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

[![Build Status](https://api.travis-ci.com/python/mypy.svg?branch=master)](https://travis-ci.com/python/mypy)
[![Documentation Status](https://readthedocs.org/projects/mypy/badge/?version=latest)](https://mypy.readthedocs.io/en/latest/?badge=latest)
[![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?
---------------

We are always happy to answer questions! Here are some good places to ask them:

- for anything you're curious about, try [gitter chat](https://gitter.im/python/typing)
- for general questions about Python typing, try [typing discussions](https://github.com/python/typing/discussions)

If you're just getting started,
[the documentation](https://mypy.readthedocs.io/en/stable/introduction.html)
and [type hints cheat sheet](https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html)
can also help answer questions.

If you think you've found a bug:

- check our [common issues page](https://mypy.readthedocs.io/en/stable/common_issues.html)
- search our [issue tracker](https://github.com/python/mypy/issues) to see if
  it's already been reported
- consider asking on [gitter chat](https://gitter.im/python/typing)

To report a bug or request an enhancement:

- report at [our issue tracker](https://github.com/python/mypy/issues)
- if the issue is with a specific library or function, consider reporting it at
  [typeshed tracker](https://github.com/python/typeshed/issues) or the issue
  tracker for that library

To discuss a new type system feature:
- discuss at [typing-sig mailing list](https://mail.python.org/archives/list/typing-sig@python.org/)
- there is also some historical discussion [here](https://github.com/python/typing/issues)


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

Mypy is a static type checker for Python.

Type checkers help ensure that you're using variables and functions in your code
correctly. With mypy, add type hints ([PEP 484](https://www.python.org/dev/peps/pep-0484/))
to your Python programs, and mypy will warn you when you use those types
incorrectly.

Python is a dynamic language, so usually you'll only see errors in your code
when you attempt to run it. Mypy is a *static* checker, so it finds bugs
in your programs without even running them!

Mypy is designed with gradual typing in mind. This means you can add type
hints to your code base slowly and that you can always fall back to dynamic
typing when static typing is not convenient.

Here is a small example to whet your appetite:

```python
number = input("What is your favourite number?")
print("Well, my favourite number is: ", number + 1)  # error: Unsupported operand types for + ("str" and "int")
```

See [the documentation](https://mypy.readthedocs.io/en/stable/introduction.html) for more examples.

In particular, see:
- [type hints cheat sheet](https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html)
- [getting started](https://mypy.readthedocs.io/en/stable/getting_started.html)


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 you can type-check the [statically typed parts] of a program like this:

    $ mypy PROGRAM

You can always use the Python interpreter to run your statically typed
programs, even if mypy reports 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


Integrations
------------

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.
* pre-commit: use [pre-commit mirrors-mypy](https://github.com/pre-commit/mirrors-mypy).

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

Additional information is available at the web site:

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

Jump straight to the documentation:

  https://mypy.readthedocs.io/

Follow along our changelog at:

  https://mypy-lang.blogspot.com/


Contributing
------------

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.

To get started with developing mypy, see [CONTRIBUTING.md](CONTRIBUTING.md).

If you need help getting started, don't hesitate to ask on [gitter](https://gitter.im/python/typing).


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

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

mypyc and compiled version of mypy
----------------------------------

[Mypyc](https://github.com/mypyc/mypyc) uses Python type hints to compile Python
modules to faster C extensions. Mypy is itself compiled using mypyc: this makes
mypy approximately 4 times faster than if interpreted!

To install an interpreted mypy instead, use:

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

To use a compiled version of a development
version of mypy, directly install a binary from
https://github.com/mypyc/mypy_mypyc-wheels/releases/latest.

To contribute to the mypyc project, check out https://github.com/mypyc/mypyc