Mypy: Static Typing for Python

Got a question?

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

for general questions about Python typing, try typing discussions
for anything you're curious about, try gitter chat

If you're just getting started,
the documentation
and type hints cheat sheet
can also help answer questions.

If you think you've found a bug:

check our common issues page
search our issue tracker to see if
it's already been reported

To report a bug or request an enhancement:

report at our issue tracker
if the issue is with a specific library or function, consider reporting it at
typeshed tracker or the issue
tracker for that library

To discuss a new type system feature:

discuss at discuss.python.org
there is also some historical discussion at the typing-sig mailing list and the python/typing repo

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)
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!

Here is a small example to whet your appetite:

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

Adding type hints for mypy does not interfere with the way your program would
otherwise run. Think of type hints as similar to comments! You can always use
the Python interpreter to run your code, even if mypy reports errors.

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.

Mypy has a powerful and easy-to-use type system, supporting features such as
type inference, generics, callable types, tuple types, union types,
structural subtyping and more. Using mypy will make your programs easier to
understand, debug, and maintain.

See the documentation for
more examples and information.

In particular, see:

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 the
repo directly:

python3 -m pip install -U git+https://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

If you are working with large code bases, you can run mypy in
daemon mode, that will give much faster (often sub-second) incremental updates:

dmypy run -- PROGRAM

You can also try mypy in an online playground (developed by
Yusuke Miyazaki).

Integrations

Mypy can be integrated into popular IDEs:

VS Code: provides basic integration with mypy.
Vim:
- Using Syntastic: in ~/.vimrc add
  let g:syntastic_python_checkers=['mypy']
- Using 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
Sublime Text: SublimeLinter-contrib-mypy
PyCharm: mypy plugin
IDLE: idlemypyextension
pre-commit: use pre-commit mirrors-mypy, although
note by default this will limit mypy's ability to analyse your third party dependencies.

Web site and documentation

Additional information is available at the web site:

https://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.

Mypyc and compiled version of mypy

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 the issue tracker at https://github.com/mypyc/mypyc

mtsokol/mypy