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.
The docs contain this example (simplified):
This works. But if we switch to "==" for comparisons, it fails:
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.Beyond the docs, this is problematic, because we can have enums that subclass the value type, e.g.
Now it actually makes a difference at runtime whether we compare with
isor==, because==also matches normal strings'up'and'down',isdoes not.Your Environment
Reproduced in Python 3.9 w/ typing_extensions and Python 3.11, using Mypy 1.0.1.