Surprising non-failure with return type

[russel]

The code:

def _validate(x:int) -> int:
    if not isinstance(x, int):
        raise TypeError('Argument must be an integer.')
    if x < 0:
        raise ValueError('Argument must be a non-negative integer.')

does not return an error with the mypy 0.4.2 that appeared in the Debian Sid repository for a while (*). The inference is that the None return is being treated as an int, which is a bit surprising.

Obviously I had intended to have:

def _validate(x:int):
    if not isinstance(x, int):
        raise TypeError('Argument must be an integer.')
    if x < 0:
        raise ValueError('Argument must be a non-negative integer.')

which also passes the mypy test.

(*) Does anyone know why the mypy package has been removed from the Debian Sid repository?

[sharmaeklavya2]

I tested this code with the latest development snapshot of mypy with the --strict-optional option but it still doesn't report any error.

[sharmaeklavya2]

@russel see #1450.

[gvanrossum]

It's a different issue. There's already an issue for it (issue#?); the problem is detecting that we fall off the end of the function body without returning a value. (It's a little trickier than you might think because of e.g. `while True: ...`.)

[rwbarton]

Yes, it's #1229; closing as duplicate of that issue.