Enum exhaustiveness checking only works using "is" comparisons, not "==", conflicting with docs

[jonasrauber]

The docs contain this example (simplified):

from enum import Enum
from typing_extensions import assert_never

class Direction(Enum):
 up = 'up'
 down = 'down'

def choose_direction(direction: Direction) -> None:
 if direction is Direction.up:
 print('Going up!')
 return
 elif direction is Direction.down:
 print('Down')
 return
 assert_never(direction)

This works. But if we switch to "==" for comparisons, it fails:

from enum import Enum
from typing_extensions import assert_never

class Direction(Enum):
 up = 'up'
 down = 'down'

def choose_direction(direction: Direction) -> None:
 if direction == Direction.up:
 print('Going up!')
 return
 elif direction == Direction.down:
 print('Down')
 return
 assert_never(direction)

error: Argument 1 to "assert_never" has incompatible type "Direction"; expected "NoReturn"

This is problematic, because the docs also have another example that uses == for comparison and implies that it would work if the second clause would be added.

def choose_direction(direction: Direction) -> None:
 if direction == Direction.up:
 print('Going up!')
 return
 assert_never(direction) # E: Argument 1 to "assert_never" has incompatible type "Direction"; expected "NoReturn"

Beyond the docs, this is problematic, because we can have enums that subclass the value type, e.g.

class Direction(str, Enum):
 up = 'up'
 down = 'down'

Now it actually makes a difference at runtime whether we compare with is or ==, because == also matches normal strings 'up' and 'down', is does not.

Your Environment

Reproduced in Python 3.9 w/ typing_extensions and Python 3.11, using Mypy 1.0.1.

[hauntsaninja]

Yeah, I was annoyed by this too. I merged my PR that has been sitting around for a long time #11521