Type-narrowing Union with Literal using comparison to Literal variable

Bug Report

Strangely, type-narrowing for unions including a Literal works for int but not datetime

To Reproduce

Mypy Playground. Copied here for convenience:

from datetime import datetime
from typing import Final, Literal

Unspecified = Literal['unspecified']
UNSPECIFIED: Final[Unspecified] = 'unspecified'

def do_thing(x: datetime | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.utcnow())  # Unexpected error!

def do_thing_2(x: int | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.real)  # No prob

Expected Behavior

No errors.

Actual Behavior

Has errors:

main.py:9: error: Item "str" of "datetime | Literal['unspecified']" has no attribute "utcnow"  [union-attr]

Your Environment

Mypy version used: 1.7.0
Mypy command-line flags: <none>
Mypy configuration options from mypy.ini (and other config files): <none>
Python version used: 3.11

I would say that this is indeed a bug. It won't be a bug in the positive case (as a class that redefines __eq__ MAY be equal to a string), but it should definitely narrow in the negative case. Here you can see that the problem manifest itself depending on if you redefined __eq__ or not:

from datetime import datetime
from typing import Final, Literal

Unspecified = Literal['unspecified']
UNSPECIFIED: Final[Unspecified] = 'unspecified'

class KK():
    
    def kk(self):
        pass

class KK2():
    
    def kk(self):
        pass
    
    def __eq__(self, other):
        return False

def do_thing(x: datetime | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.utcnow())  # Unexpected error!

def do_thing_2(x: int | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.real)  # No prob
        
def do_thing_3(x: KK | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.kk)  # No prob
        
def do_thing_4(x: KK2 | Unspecified) -> None:
    if x != UNSPECIFIED:
        print(x.kk)  # Unexpected error!

Copying over my smaller reproducer from #18569:

from typing import Literal

class A:
    def __eq__(self, other: object) -> bool:  # necessary
        return isinstance(other, A)

def f(v: A | Literal["text"]) -> A | None:
    if v == "text":
        reveal_type(v)  # N: Revealed type is "Union[__main__.A, Literal['text']]"
        return None
    else:
        reveal_type(v)  # N: Revealed type is "Union[__main__.A, Literal['text']]"
        return v

Better narrowing with custom equality (python#20643)

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Type-narrowing Union with Literal using comparison to Literal variable #16465

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Type-narrowing Union with Literal using comparison to Literal variable #16465

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vnmabus commented on Nov 24, 2023

A5rocks commented on Jan 30, 2025

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