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 0.620 released

13 July 2018: Mypy 0.620 was released. It adds support for dataclasses, improvements to overloads, better support for PEP 561 and several smaller improvements. Read the blog post for more details. -Ivan Levkivskyi

Mypy 0.610 released

8 June 2018: Mypy 0.610 was released. It adds support for dmypy run and a host of other small improvements. Read the blog post to see what's changed. (Note that we're once again starting to link blog posts here.) -Michael J. Sullivan

Dropbox releases PyAnnotate

15 November 2017: Dropbox has released an open source tool for automatically generating type annotations based on runtime type collection. See the blog post for details. -Guido van Rossum

For more mypy news see the mypy blog.

24 October 2017: We're increasing mypy's release frequency to about once per three weeks. We no longer will link here to each separate release blog post; just check the mypy blog for news. Two new versions, 0.530 and 0.540, were released since the last news item below. -Guido van Rossum

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