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MyPy gives error “Missing return statement” even when all cases are tested #4223
Description
I haven't received any answers to the following StackOverflow post, so I thought I'd also post it here. I'm not sure if it's an actual bug or if it's just considered good practice to always have something like else: raise Exception at the end of if / elif block.
I am getting a MyPy error "Missing return statement", even when I check for all possible cases inside a function.
For example, in the following code, MyPy is still giving me an error "9: error: Missing return statement", even though color can only be Color.RED, Color.GREEN, or Color.BLUE, and I test all those cases!
class Color(enum.IntEnum):
RED: int = 1
GREEN: int = 2
BLUE: int = 3
def test_enum(color: Color) -> str:
if color == Color.RED:
return "red"
elif color == Color.GREEN:
return "green"
elif color == Color.BLUE:
return "blue"
Activity
elazarg commentedon Nov 8, 2017
This code is fragile; if
Colorwill change, it will silently break. The right way IMO is to addassert Falseat the end. This will also silence mypy.To the point, I don't think it's worth the special casing inside mypy. If we'll ever have singleton types, it might be possible to make it work out of the box.
ilevkivskyi commentedon Nov 13, 2017
PEP 484 actually explicitly specifies that examples like this should be supported. I am not sure if we already have an issue for this, so I will keep this open.
ilevkivskyi commentedon Nov 13, 2017
Just for reference, the relevant section is https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions
Added sentinel for missing return value - see python/mypy#4223 (comment)
ilevkivskyi commentedon Jul 6, 2019
I think we can now track this in #6366, which is a more general issue.