Support Union narrowing with Ellipsis

[antdking]

Effectively the same as this issue from mypy

Ellipsis is a constant in Python, so we're able to do identity checks on it for narrowing.

This allows supporting using Ellipsis as a sentinel value where None is an accepted input.

basic example:

import types

# crude, but valid in more complex cases, such as a fallback when accessing a store
def get_val(default=...) -> int | types.EllipsisType:
    return default

x = get_val()

if x is not ...:
    reveal_type(x)

reveal_type output:

Pylance: Type of "x" is "int | ellipsis"
mypy(note): Revealed type is "builtins.int"

[erictraut]

Moving to pyright, since this is a core type checking issue.

[erictraut]

This doesn't strike me as a common idiom in Python, but since support for this type guard was added to mypy recently, I think it's reasonable to add support in pyright as well.

This will be addressed in the next release.

[antdking]

Moving to pyright, since this is a core type checking issue.

Woops, sorry about that. Glad it's clean to move over.

This doesn't strike me as a common idiom in Python

So you have another usecase (outside of "mypy did it"), I did more digging.
Our wider usecase is related to Pydantic, and how it represents Required-Nullable fields. Though I think you'll only encounter it when building dynamic models.

Thanks for the quick fix on this!

[erictraut]

This is addressed in pyright 1.1.288, which I just published. It will also be included in a future release of pylance.