Mypy is an optional static type checker for Python. You can add type hints to your Python programs using the upcoming standard for type annotations introduced in Python 3.5 beta 1 (PEP 484), and use mypy to type check them statically. Find bugs in your programs without even running them!

The type annotation notation has also been backported to earlier Python 3.x versions. Mypy programs are valid Python 3.x and you use a normal Python interpreter to run them. There is essentially no performance overhead when using mypy, since mypy does not introduce runtime type checking.

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.

You need Python 3.2 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

For other Linux flavors, OS X and Windows, packages are available at

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

Mypy can be installed using pip:

$ pip install mypy-lang

Depending on your configuration, you may have to use pip3 instead:

$ pip3 install mypy-lang

If you want to contribute, first clone the mypy git repository:

$ git clone https://github.com/JukkaL/mypy.git

Run the supplied setup.py script to install mypy:

$ python3 setup.py install

Replace python3 with your Python 3 interpreter. You may have to do the above as root. For example, in Ubuntu and Mac OS X:

$ sudo python3 setup.py install

This installs the mypy script and dependencies, including the typing module, 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

Now, on a Unix-like system, you can type check a program like this:

$ mypy PROGRAM

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

You can always use a Python interpreter to run your statically typed programs, even if they have type errors:

$ python3 PROGRAM

Documentation and additional information is available at the web site:

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

First install any additional dependencies needed for testing:

$ pip install -r test-requirements.txt

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

$ python3 tests.py

You can also run tests without having to run setup.py first by setting up the Python module search path suitably:

$ export PYTHONPATH=PREFIX/mypy:PREFIX/mypy/lib-typing/3.2
$ python3 tests.py

Replace PREFIX with the path where you have the repository cloned.

You can also run the type checker for manual testing now without installing anything by running scripts/mypy:

$ python3 PREFIX/mypy/scripts/mypy PROGRAM

To run the linter:

$ ./lint.sh

Mypy is work in progress and is not yet production quality (though mypy development is already done using mypy!).

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

• Python 2.x support not usable yet
• 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 (initially 3.x, and later 2.7).

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

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

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. Contact the developers to join the project, or just start coding and send pull requests! There are tasks for contributors of all experience levels.

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