The mock tests in PaaS I/O in Python on Exercism are returning non-None values from __exit__ of IO classes . However, based on the type hints defined in the Python standard library, such non-None values are considered “impossible”. More info on the reasoning can be found at The return type hint of __exit__ should be Optional[bool] instead of None · Issue #103573 · python/cpython · GitHub
In short, the __exit__ method should never return any value other than None in the mock tests (which are currently returning None, True and False under different conditions). Currently these tests violate what the IO classes are supposed to return per the defined type hints in the Python library.
Personally I found the tests very confusing, so I thought I’d report this in case someone else ran into the same pains 
(The corresponding Discord thread.)