As mypy is a static analyzer, or a lint-like tool, the type annotations are just hints for mypy and don't interfere when running your program. ... Mypy has a powerful and easy-to-use type system with modern features such as type inference, generics, callable types, tuple types, union types, and structural subtyping.

What is 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.” A little background on the Mypy project.

Should I use mypy?

Conclusion. Overall, the experience of using mypy (and the PEP-484 type system) has been awesome, and we feel that adopting mypy has been a big advance for the Zulip project. It improves readability, catches bugs in code without running it, has very few false positives, and hasn't come with significant downsides.