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
for anything you're curious about, try
If you're just getting started,
and
can also help answer questions.
If you think you've found a bug:
check our
search our
to see if it's already been reported
To report a bug or request an enhancement:
report at
if the issue is with a specific library or function, consider reporting it at
or the issue tracker for that library
To discuss a new type system feature:
discuss at
there is also some historical discussion at the
and the
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 (
) 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
for more examples and information.
In particular, see:
Quick start
Mypy can be installed using pip:
python3 -m pip install -U mypyIf 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.gitNow you can type-check the
of a program like this:
mypy PROGRAMYou can always use the Python interpreter to run your statically typed programs, even if mypy reports type errors:
python3 PROGRAMIf you are working with large code bases, you can run mypy in
, that will give much faster (often sub-second) incremental updates:
dmypy run -- PROGRAMYou can also try mypy in an
(developed by Yusuke Miyazaki).
Integrations
Mypy can be integrated into popular IDEs:
VS Code: provides
with mypy.
Vim: Using
: in ~/.vimrc add let g:syntastic_python_checkers=['mypy']
Using
: 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
Sublime Text:
PyCharm:
IDLE:
pre-commit: use
, 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:
Jump straight to the documentation:
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
.
Mypyc and compiled version of mypy
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 mypyTo 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