Type not narrowed correctly for `if X in collection`

[frexvahi]

def foobar(foo: Union[str, float]):
    if foo in ['fizz', 'buzz']:
        reveal_type(foo)
    else:
        pass

The revealed type is Union[builtins.str, builtins.float] but should be builtins.str

[ilevkivskyi]

Than you for reporting this! I have also encountered situations where this could be useful. It is not super easy to implement, but I think this is a reasonable feature to support (especially for invariant collections, where the type is known precisely).

As a workaround, you could add assert isinstance(foo, str) in the if-branch, mypy will recognize this.

[elazarg]

Related #2357 #1203

[frexvahi]

I just realised: there might also have to be a special case for testing if a float is in a set of ints, or any other situation where a == b can be true even though type(a) == type(b) is not.

[JukkaL]

Reopening, since the original implementation was reverted due to some issues (#17864).

[Jordandev678]

@JukkaL want me to pick this up?

[JukkaL]

@Jordandev678 Sure! The original PR can be resubmitted once the issues have been addressed.