Mypy: Optional Static Typing for Python

Got a question? File an issue!

We don't have a mailing list; but we are always happy to answer questions on gitter chat or filed as issues in our trackers:

• mypy tracker for mypy isues
• 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 to your Python programs using the standard for type annotations introduced in Python 3.5 (PEP 484), and use mypy to type check them statically. Find bugs in your programs without even running them!

The type annotation standard has also been backported to earlier Python 3.x versions. Mypy supports Python 3.3 and later.

For Python 2.7, you can add annotations as comments (this is also specified in PEP 484).

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:

from typing import Iterator

def fib(n: int) -> Iterator[int]:
    a, b = 0, 1
    while a < n:
        yield a
        a, b = b, a + b

Mypy is in development; some features are missing and there are bugs. See 'Development status' below.

Requirements

You need Python 3.3 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

http://www.python.org/getit/

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 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

Web site and documentation

Documentation and additional information is available at the web site:

http://www.mypy-lang.org/

Or you can jump straight to the documentation:

http://mypy.readthedocs.io/

Troubleshooting

Depending on your configuration, you may have to run pip3 like this:

$ python3 -m pip install -U mypy

Except on Windows, it's best to always use the --fast-parser option to mypy; this requires installing typed-ast:

$ python3 -m pip install -U typed-ast

If the mypy command isn't found after installation: After either pip3 install or setup.py install, the mypy script and dependencies, including the typing module, will be installed to system-dependent locations. Sometimes the script directory will not be in PATH, and you have to add the target directory to PATH manually or create a symbolic link to the script. In particular, on Mac OS X, the script may be installed under /Library/Frameworks:

/Library/Frameworks/Python.framework/Versions/<version>/bin

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

Working with virtualenv

If you are using virtualenv, make sure you are running a python3 environment. Installing via pip3 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

From the mypy directory, use pip to install mypy:

$ cd mypy
$ python3 -m pip install -U .

Replace 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.

The mypy wiki contains some useful information for contributors:

https://github.com/python/mypy/wiki/Developer-Guides

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")

Running tests and linting

First install any additional dependencies needed for testing:

$ python3 -m pip install -U -r test-requirements.txt

To run all tests, run the script runtests.py in the mypy repository:

$ ./runtests.py

Note that some tests will be disabled for older python versions.

This will run all tests, including integration and regression tests, and will type check mypy and verify that all stubs are valid.

You can run a subset of test suites by passing positive or negative filters:

$ ./runtests.py lex parse -x lint -x stub

For example, to run unit tests only, which run pretty quickly:

$ ./runtests.py unit-test pytest

The unit test suites are driven by a mixture of test frameworks: mypy's own myunit framework, and pytest, which we're in the process of migrating to. For finer control over which unit tests are run and how, you can run py.test or scripts/myunit directly, or pass inferior arguments via -a:

$ py.test mypy/test/testcheck.py -v -k MethodCall
$ ./runtests.py -v 'pytest mypy/test/testcheck' -a -v -a -k -a MethodCall

$ PYTHONPATH=$PWD scripts/myunit -m mypy.test.testlex -v '*backslash*'
$ ./runtests.py mypy.test.testlex -a -v -a '*backslash*'

You can also run the type checker for manual testing without installing anything by setting up the Python module search path suitably (the lib-typing/3.2 path entry is not needed for Python 3.5 or when you have manually installed the typing module):

$ export PYTHONPATH=$PWD:$PWD/lib-typing/3.2
$ python<version> -m mypy PROGRAM.py

You can add the entry scripts to PATH for a single python3 version:

$ export PATH=$PWD/scripts
$ mypy PROGRAM.py

You can check a module or string instead of a file:

$ mypy PROGRAM.py
$ mypy -m MODULE
$ mypy -c 'import MODULE'

To run the linter:

$ ./runtests.py lint

Coverage reports

There is an experimental feature to generate coverage reports. To use this feature, you need to pip install -U lxml. This is an extension module and requires various library headers to install; on a Debian-derived system the command apt-get install python3-dev libxml2-dev libxslt1-dev may provide the necessary dependencies.

To use the feature, pass e.g. --txt-report "$(mktemp -d)".

Development status

Mypy is work in progress and is not yet production quality, though mypy development has been done using mypy for a while!

Here are some of the more significant Python features not supported right now (but all of these will improve):

• properties with setters not supported
• limited metaclass support
• only a subset of Python standard library modules are supported, and some only partially
• 3rd party module support is limited

The current development focus is to have a good coverage of Python features and the standard library (both 3.x and 2.7).

Issue tracker

Please report any bugs and enhancement ideas using the mypy issue tracker:

https://github.com/python/mypy/issues

Feel free to also ask questions on the tracker.

Help wanted

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, check out the difficulty/easy label.

For more details, see the file CONTRIBUTING.md.

License

Mypy is licensed under the terms of the MIT License (see the file LICENSE).