README.md

Mypy: Static Typing for Python

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
• for general questions about Python typing, try typing discussions

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
• consider asking on gitter chat

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:

• type hints cheat sheet
• getting started
• list of error codes

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
# or if you don't have 'git' installed
python3 -m pip install -U https://github.com/python/mypy/zipball/master

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 (developed by Yusuke Miyazaki). 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

Integrations

Mypy can be integrated into popular IDEs:

• 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
• Atom: linter-mypy
• PyCharm: mypy plugin (PyCharm integrates its own implementation of PEP 484)
• VS Code: provides basic integration with mypy.
• pre-commit: use pre-commit mirrors-mypy.

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.

If you need help getting started, don't hesitate to ask on gitter.

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 https://github.com/mypyc/mypyc