Mypy: MyPy gives error “Missing return statement” even when all cases are tested

[ostrokach]

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.

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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"

[elazarg]

This code is fragile; if Color will change, it will silently break. The right way IMO is to add assert False at 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]

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]

Just for reference, the relevant section is https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions

[ilevkivskyi]

I think we can now track this in https://github.com/python/mypy/issues/6366, which is a more general issue.