Use a static type checker called mypy. In libs, I think it's very important to have type hints, so that when users are going to use the lib, code linters can help, knowing exactly what kind of parameters to receive, and what the return type is.

The ideal solution would be to add type hints as specified by PEP 484, PEP 526, PEP 544, PEP 586, PEP 589, and PEP 591 directly on your code.