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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 0.3 released

19 Feb 2016: Mypy 0.3 was released. It's focused on Python 2 support and better PEP 484 compatibility, but it's much improved in general. Read the blog post to see what's changed. Note that the mypy GitHub repo was moved to a new URL (https://github.com/python/mypy). -Jukka

Mypy 0.2 released

5 Apr 2015: Mypy 0.2 was released. It's focused on PEP 484 (Type Hinting) draft compatibility and more complete Python feature support. Blog post -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