> Use beartype to assure the quality of Python code beyond what tests alone can assure. If you have yet to test, do that first with a pytest-based test suite, tox configuration, and continuous integration (CI). If you have any time, money, or motivation left, annotate callables and classes with PEP-compliant type hints and decorate those callables and classes with the @beartype.beartype decorator.

Don't get me wrong, I think static type checking is great. Now if you need to add a decorator on top of each class and function AND maintain 100% code coverage, well that does not sound like "zero-cost" to me. I can hardly think of a greater cost just to continue dynamically typing your code and maintain guarantees about external dependencies with no type hints.