mypy

mypy is an optional static type checker for Python that aims to combine the benefits of dynamic (duck) typing and static typing. Compile-time type checking finds bugs with less debugging, and type declarations act as machine-checked documentation.

Programs can be developed with dynamic typing and have static typing added once the code has matured, and existing Python code can be migrated progressively to static typing.

mypy is open source and actively developed on GitHub as part of the Python organisation.