Incompatible types: Union[bool, A] and Optional[A]

[carmocca]

Bug Report

from typing import Union, Optional


class Foo:
    ...
    
    
class Bar(Foo):
    ...
    
    
class Thing:
    thing: Optional[Foo]
    
    def errors(self, x: Union[bool, Foo]) -> None:
        if x is True:
            self.thing = Bar()
        elif x is False:
            self.thing = None
        else:
            self.thing = x
            
    def works(self, x: Union[bool, Foo]) -> None:
        if isinstance(x, bool):
            self.thing = Bar() if x else None
        else:
            self.thing = x

I would expect that errors and works are equal, but errors produces an error.

Sorry if this is a duplicate, I tried googling it!

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.10&gist=f17cd6e5962f2edd358143dcce83ab51

Expected Behavior

No error

Actual Behavior

main.py:21: error: Incompatible types in assignment (expression has type "Union[bool, Foo]", variable has type "Optional[Foo]")
Found 1 error in 1 file (checked 1 source file)

Your Environment

See link above - can be reproduced in all versions

[erictraut]

Mypy doesn't currently narrow bool types when the value is compared against True or False, so the type of x within the else clause is Union[bool, Foo]. This new capability would need to be added to make the above code type check without error.

For now, you could work around it using the technique you identified, or you can add a assert not isinstance(x, bool) statement in the else clause of the errors method.

[hauntsaninja]

Thanks, this will be fixed by #10389