Mypy is a Python variant which lets you add type annotations to Python programs and type check them statically to find programming errors before running them. Mypy allows you to seamlessly mix dynamic and static typing in your programs. Mypy programs are valid Python 3.x and you can use a normal Python interpreter to run them.

Here is a small example:

import typing

def fib(n: int) -> None:
    a, b = 0, 1
    while a < n:
        print(a)
        a, b = b, a+b

Mypy is work in progress; many features are missing and there are still bugs. See 'Development status' below.

The long-term goal of the project is to also develop an optimizing compiler that can compile mypy programs to efficient native code. The compiler is still early in development.

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/

If you have git, first clone the mypy git repository:

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

Alternatively, you can download the latest development version as a zip archive from this URL:

https://github.com/JukkaL/mypy/archive/master.zip

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 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 and run programs:

$ mypy PROGRAM

In Windows, the script is generally installed in \PythonNN\Scripts. So, type check and run a program like this (replace \Python33 with your Python installation path):

C:\>\Python33\python \Python33\Scripts\mypy PROGRAM

Mypy programs are valid Python programs, so you can use the Python interpreter to run them without type checking:

$ python3 PROGRAM

Documentation and additional information is available at the web site:

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

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

$ python3 tests.py

Mypy is work in progress and is not yet production quality (though mypy development is already done in 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 yet usable
• no properties
• no static or class methods
• int is not compatible with float
• tuples are not well-supported as immutable lists
• somewhat limited operator overloading
• only a subset of Python standard library modules are supported, and some only partially
• very limited metaclass support

Some mypy-specific features are also not supported or only partially supported, including these:

• function overloading does not work properly in all cases, including some instances of method overriding, and keyword arguments
• no 'Dynamic' classes
• there is no way to use dynamic typing by default for top-level code

The current development focus is to support a good subset of Python features (both 2.x and 3.x).

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

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

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 skill levels.

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