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 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.

What's new

Mypy 0.812 released

19 February 2021: Mypy 0.812 was released. This release fixes a regression in module finding behavior and adds the --exclude flag for excluding certain paths when searching for modules. Read the blog post for the details. -Jukka Lehtosalo

Mypy 0.800 released

22 January 2021: Mypy 0.800 was released. This release adds Python 3.9 support, typing usability improvements (PEP 585 and PEP 604), and other features and bug fixes. Read the blog post for the details. -Ivan Levkivskyi

Mypy 0.790 released

9 October 2020: Mypy 0.790 was released. Read the blog post for the details. -Jukka Lehtosalo

Mypy 0.780 released

3 June 2020: Mypy 0.780 was released. Read the blog post for the details. -Ivan Levkivskyi

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