Mypy: Static Typing for Python
Got a question?
We are always happy to answer questions! Here are some good places to ask them:
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:
To report a bug or request an enhancement:
To discuss a new type system feature:
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:
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
