Type hints and optimized mode

[naglis]

While reviewing a PR I came across a somewhat interesting case - the validity of the return type depending on whether Python is running in optimized mode (starting Python with -O or PYTHONOPTIMIZE environment variable being set). E.g.:

import typing


def foo() -> typing.NoReturn:
    assert False


print(foo())

the return type of foo is not valid in optimized mode since the assert statement will be removed and calling foo would return None.

Checking the snippet with mypy (version 1.8.0) does not report/warn about the issue.

I have searched in the documentation, PEPs 483 and 484, in this repository and on mypy, but could not find optimized mode being mentioned. How should such a case be handled? Should the type hint of the return type of foo be somehow dynamic depending on __debug__? Or are type hints always considered in non-optimized mode?

[sobolevn]

Just use raise AssertionError instead of assert

[Daverball]

This is something a linter should warn you about and in fact flake8-bugbear does exactly that with B011. I don't think it's reasonable to expect a type checker to worry about what may or may not change its behavior in optimized builds.