Mypy can't deal with exclusive options implied by a Union

[ariciputi]

Hi,
I have a use case in which (AFAICT) mypy is not able to deal with the exclusive options given by a Union.

• Are you reporting a bug, or opening a feature request?

A bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

Here it is a minimal code snippets:

from typing import Collection, Union

def function(a: Union[bool, Collection[str]], b: str) -> int:
    if a is True:
        return 1
    if a is False:
        return 2

    if b in a:
        return 3

    return 4

• What is the actual behavior/output?

example.py:10: error: Unsupported right operand type for in ("Union[bool, Collection[str]]")

• What is the behavior/output you expect?

This is a perfectly valid Python code, so mypy shouldn't complain.

• What are the versions of mypy and Python you are using?

$ mypy --version
mypy 0.720

Do you see the same issue after installing mypy from Git master?

Haven't tried

• What are the mypy flags you are using? (For example --strict-optional)

None

Hope you can help.

[JukkaL]

Thanks for the report! It might be a reasonable for mypy to infer a narrower type after testing a boolean. Say, after if bool_value: return (or if bool_value is True: return) we might infer Literal[False] as the type of bool_value. This seems closely related to #6366 but a slightly different use case.

@Michael0x2a What do you think about this?

[ariciputi]

Interestingly enough the following works with latest version of mypy:

from typing import Collection, Union
from enum import Enum

Sentinel = Enum("Sentinel", ("TRUE", "FALSE"))

def function(a: Union[Sentinel, Collection[str]], b: str) -> int:
    if a is Sentinel.TRUE:
        return 1
    if a is Sentinel.FALSE:
        return 2

    if b in a:
        return 3

    return 4

but is failing with the very same error message as above with version 0.710

HTH

[ilevkivskyi]

I think this is a duplicate of #6113 (treating bool as equivalent to Union[Literal[True], Literal[False]]). I am going to raise priority there and update the title.