Mypy doesn't throw when returning a variable that is only defined conditionally

[ckarnell]

This looks like a bug to me:

def test_func() -> int:
    y: bool = False
    if y:
        x = 5
    return x # No error

When a variable is defined behind a conditional with type bool, then mypy doesn't flag that the variable might not exist outside of the condition. I noticed that mypy can tell that the variable is False, then it will at least say "cannot determine type of x", but not when the variable is just a bool.

This will very often be a runtime error, so I would expect mypy to throw here. Is this behavior intended?

[msullivan]

Yeah, this is #2400. It's a feature request, technically, but seems important. We'd like to get it done.

[ckarnell]

Awesome, thanks for the quick response @msullivan