Strict optional doesn't recognize "None in (...)" tests

[JukkaL]

A user reported code which basically boils down to this:

class A:
    a: Optional[int]
    b: Optional[int]
    ...
    def f(self) -> None:
        if None in (self.a, self.b):
            return
        print(self.a + self.b)  # ok at runtime but mypy error currently

Mypy doesn't recognize the None in (...) test as a legitimate None test.

The code seems a little unusual but if we see this frequently enough it might be worth supporting at some point.

[gvanrossum]

Hm, I think it's a weird idiom -- a little hard to understand when you see it first.

[refi64]

FWIW, doesn't this end up basically being self.a == None and self.b == None? I'm not sure if it would be entirely safe, since it runs into the normal issues of using == in None tests (e.g. custom __eq__ overloads).

[gvanrossum]

Sure (with 'or' though), but AFAIK we allow that elsewhere too, so that doesn't concern me.

[ilevkivskyi]

There is a similar, but more useful idiom (that is used several times in mypy itself):

x: Optional[int]
cont: Container[int]  # note, non-optional content type
if x in cont:
    reveal_type(x) # this can be only 'int'

[ilevkivskyi]

The above is probably only safe for built-in containers, where we know the behavior of __contains__, but if we allow this at least for list and dict, this will be already helpful for --strict-optional.

[JukkaL]

Here's another example reported by @memery-imb in #8279:

def mypy_subtract(nulled: bool) -> float:
    if nulled:
        returnable = [None, None]
    else:
        returnable = [1.0, 2.0]

    if None in returnable:
        raise Exception

    return returnable[0] - returnable[1]