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

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.

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

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).

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

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'

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.

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]

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Strict optional doesn't recognize "None in (...)" tests #2980

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Strict optional doesn't recognize "None in (...)" tests #2980

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gvanrossum commented on Mar 8, 2017

refi64 commented on Mar 8, 2017

gvanrossum commented on Mar 8, 2017

ilevkivskyi commented on Oct 7, 2017

ilevkivskyi commented on Oct 7, 2017

JukkaL commented on Jan 15, 2020

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