Return value expected on def f() -> Optional[Type]: with plain return

[darjus]

mypy is expecting me to explicitly return None instead of return, but I'd expect that to be inferred as there's little point in me telling it i return None

def f(i: int) -> Optional[int]:
    if i > 3:
        return  # fails but return None works
    return i * 3

[gvanrossum]

Actually this is intentional -- it's a code smell and even called out in PEP 8 (https://www.python.org/dev/peps/pep-0008/#programming-recommendations, "Be consistent in return statements.")

[darjus]

Thanks for explaining this @gvanrossum . I've read pep8 several times before but didn't quite follow this specific recommendation, moreover pep8 command does not complain (at least my version) about this.
The actual message is error: Return value expected which threw me off.
Should we invest in a more descriptive message and or documentation (I'm willing to submit a pr for this), or you think this is sufficient?

[gvanrossum]

Sadly the pep8 tool's name is an insult to the intentions of the PEP 8 document.

[mlissner]

I'm all for eliminating code smells, but I just struggled with this error a bit because I'm not in the state of mind to fix pep8 stuff when I run mypy.

I'd suggest removing this warning since it isn't about typing. If the function returns None and that's what the signature calls for, then mypy shouldn't complain otherwise.