I recently added type annotations to my Python 3.7 project and found it very helpful. Just the process of adding the annotations caught a handful of bugs! Some

I recently added type annotations to my Python 3.7 project and found it very helpful. Just the process of adding the annotations caught a handful of bugs! Some things I found useful: 1. from __future__ import annotations As I mentioned in another comment, this lets you write annotations naturally without worrying about when something gets defined. 2. typing.TYPE_CHECKING is only True while type checking. This allows you to conditionally import files that would otherwise cause a circular dependency so that you can use the symbols for annotations. 3. def foo(bar: str = None) -> str If an argument defaults to None, then mypy will recognize that its type is actually Optional[whatever]. So in the example above, bar is an Optional[str]. (I'm not sure if this is mypy-specific.) Optional[T] is equivalent to Union[T, None]. 4. You can make typedefs easily. MyType = Union[Sequence[str], Dict[str, int]] Makes writing complicated annotations easier.