Mypy on Twitter is @mypyproject

Why mypy?

Compile-time type checking: Static typing makes it easier to find bugs with less debugging.
Easier maintenance: Type declarations act as machine-checked documentation. Static typing makes your code easier to understand and easier to modify without introducing bugs.
Grow your programs from dynamic to static typing: You can develop programs with dynamic typing and add static typing after your code has matured, or migrate existing Python code to static typing.

mypy

Mypy is an experimental optional static type checker for Python that aims to combine the benefits of dynamic (or "duck") typing and static typing. Mypy combines the expressive power and convenience of Python with a powerful type system and compile-time type checking. Mypy type checks standard Python programs; run them using any Python VM with basically no runtime overhead.

Mypy is still in development. Most Python features are supported.

What's new

Mypy 0.510 released

5 May 2017: Mypy 0.510 was released. It adds support for overloads in source files and more flexible callables, but these are just a few of the improvements and fixes. Read the blog post to see what's changed. -Jukka

Mypy 0.501 released

1 Mar 2017: Mypy 0.501 was released. It switches to a new default parser and supports Python 3.6 features by default. These are also many other improvements and fixes. Read the blog post to see what's changed. -Jukka

Mypy 0.470 released

13 Jan 2017: Mypy 0.470 was released. This release release switches to the PyPI package name "mypy" instead of "mypy-lang" and switches to a new version numbering scheme. It also includes many other improvements and fixes. Read the blog post to see what's changed. -Jukka

Mypy 0.4.6 released

21 Nov 2016: Mypy 0.4.6 was released. This release adds support for generic type aliases, missing return statement detection and self types. It also improves type checking of import cycles and includes many other improvements and fixes. Read the blog post to see what's changed. -Jukka

Older news

Seamless dynamic and static typing

From Python...

def fib(n):
    a, b = 0, 1
    while a < n:
        yield a
        a, b = b, a+b

...to statically typed Python

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

Migrate existing code to static typing, a function at a time. You can freely mix static and dynamic typing within a program, within a module or within an expression. No need to give up dynamic typing — use static typing when it makes sense. Often just adding function signatures gives you statically typed code. Mypy can infer the types of other variables.

Python syntax

Mypy type checks programs that have type annotations conforming to PEP 484. Getting started is easy if you know Python. The aim is to support almost all Python language constructs in mypy.

Powerful type system

Mypy has a powerful, modern type system with features such as bidirectional type inference, generics, callable types, abstract base classes, multiple inheritance and tuple types.

Access to Python libs

Many commonly used libraries have stubs (statically typed interface definitions) that allow mypy to check that your code uses the libraries correctly.

Learn more