Type narrowing with identity check

[hamdanal]

Is your feature request related to a problem? Please describe.

According to the pyright documentation https://microsoft.github.io/pyright/#/type-concepts-advanced?id=type-guards, narrowing with identity check using the is operator is supported for booleans and enumerations. It would be nice to also be able to narrow on identity checks with other literals.

Describe the solution you’d like

Currently pyright thinks that x could be None in the if branch:

from random import choice
from typing import Final

ONE: Final = 1
x: int | None = choice((1, None))
if x is ONE:
    reveal_type(x)  # mypy: "int", pyright: "Literal[1] | None"
else:
    reveal_type(x)  # mypy: "int | None", pyright: "Literal[1] | None"

I expect the type in the if branch to be narrowed to "Literal[1]". It should be safe to do this kind of narrowing here.

[erictraut]

The is operator checks whether the two operands are the same object, not whether the two objects have equal value. You should use == here instead if you want your code to be robust.

The code in your example happens to work because the Python runtime interns certain small int objects, effectively treating all 1 instances as the same object. It doesn't necessarily do this for all int values. It also interns some short string values. However, this behavior is an implementation detail that may not hold for other implementations or versions of Python. Your code should not rely on it. Use == instead.

The main purpose of a type checker is to help you write robust, maintainable code. This coding pattern does the opposite, so it's unlikely that we'd ever support the requested feature.

[erictraut]

On second thought, if you change your code to choice(ONE, None), it would avoid the unsafe assumption.

I also initially missed the fact that mypy implements this type narrowing form.

Reopening for reconsideration.