Mypy: Optional Static Typing for Python
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We don't have a mailing list; but we are always happy to answer questions on gitter chat. If you are sure you've found a bug please search our issue trackers for a duplicate before filing a new issue:
- mypy tracker for mypy issues
- typeshed tracker for issues with specific modules
- typing tracker for discussion of new type system features (PEP 484 changes) and runtime bugs in the typing module
What is mypy?
Mypy is an optional static type checker for Python. You can add type hints (PEP 484) to your Python programs, and use mypy to type check them statically. Find bugs in your programs without even running them!
You can mix dynamic and static typing in your programs. You can always fall back to dynamic typing when static typing is not convenient, such as for legacy code.
Here is a small example to whet your appetite (Python 3):
from typing import Iterator def fib(n: int) -> Iterator[int]: a, b = 0, 1 while a < n: yield a a, b = b, a + b
See the documentation for more examples.
For Python 2.7, the standard annotations are written as comments:
def is_palindrome(s): # type: (str) -> bool return s == s[::-1]
Mypy is in development; some features are missing and there are bugs. See 'Development status' below.
You need Python 3.4 or later to run mypy. You can have multiple Python versions (2.x and 3.x) installed on the same system without problems.
In Ubuntu, Mint and Debian you can install Python 3 like this:
$ sudo apt-get install python3 python3-pip
For other Linux flavors, OS X and Windows, packages are available at
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, if Python on your system is configured properly (else see "Troubleshooting" below), you can type-check the statically typed parts of a program like this:
$ mypy PROGRAM
You can always use a Python interpreter to run your statically typed programs, even if they have type errors:
$ python3 PROGRAM
IDE & Linter Integrations
Mypy can be integrated into popular IDEs:
- Vim: vim-mypy
- Emacs: using Flycheck and Flycheck-mypy
- 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.
Web site and documentation
Documentation and additional information is available at the web site:
Or you can jump straight to the documentation:
Depending on your configuration, you may have to run
$ python3 -m pip install -U mypy
This should automatically install the appropriate version of mypy's parser, typed-ast. If for some reason it does not, you can install it manually:
$ python3 -m pip install -U typed-ast
mypy command isn't found after installation: After
python3 -m pip install, the
mypy script and
dependencies, including the
typing module, will be installed to
system-dependent locations. Sometimes the script directory will not
PATH, and you have to add the target directory to
manually or create a symbolic link to the script. In particular, on
Mac OS X, the script may be installed under
In Windows, the script is generally installed in
\PythonNN\Scripts. So, type check a program like this (replace
\Python34 with your Python installation path):
C:\>\Python34\python \Python34\Scripts\mypy PROGRAM
If you are using
make sure you are running a python3 environment. Installing via
in a v2 environment will not configure the environment to run installed
modules from the command line.
$ python3 -m pip install -U virtualenv $ python3 -m virtualenv env
Quick start for contributing to mypy
If you want to contribute, first clone the mypy git repository:
$ git clone --recurse-submodules https://github.com/python/mypy.git
If you've already cloned the repo without
you need to pull in the typeshed repo as follows:
$ git submodule init $ git submodule update
Either way you should now have a subdirectory
typeshed containing a
clone of the typeshed repo (
From the mypy directory, use pip to install mypy:
$ cd mypy $ python3 -m pip install -U .
python3 with your Python 3 interpreter. You may have to do
the above as root. For example, in Ubuntu:
$ sudo python3 -m pip install -U .
Now you can use the
mypy program just as above. In case of trouble
see "Troubleshooting" above.
Working with the git version of mypy
mypy contains a submodule, "typeshed". See http://github.com/python/typeshed. This submodule contains types for the Python standard library.
Due to the way git submodules work, you'll have to do
git submodule update typeshed
whenever you change branches, merge, rebase, or pull.
(It's possible to automate this: Search Google for "git hook update submodule")
The basic way to run tests:
$ pip3 install -r test-requirements.txt $ ./runtests.py
For more on the tests, see Test README.md
Mypy is alpha software, but it has already been used in production for well over a year at Dropbox, and it has an extensive test suite.
See the roadmap if you are interested in plans for the future.
Please report any bugs and enhancement ideas using the mypy issue tracker:
Feel free to also ask questions on the tracker.
Any 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. If you're just getting started, ask on the gitter chat for ideas of good beginner issues.
For more details, see the file CONTRIBUTING.md.
Mypy is licensed under the terms of the MIT License (see the file LICENSE).