Improve ellipsis type

[aaronmondal]

The current ellipsis = TypeVar('ellipsis') doesn't behave well with typechecking. It's also unclear in the Markdown docs what ellipsis is supposed to be. It's kind of clear from the context, but not compatible with "real" python types:

from types import EllipsisType
from typing import Protocol, TypeVar

ellipsis = TypeVar('ellipsis')  # This is the current implementation.

class BadProto(Protocol):
    def f(self, ell: ellipsis) -> None: ...  # Error: TypeVar "ellipsis" appears onlly once in generic function signature


class GoodProto(Protocol):
    def f(self, ell: EllipsisType) -> None: ...  # Ok, no error raised.


def some_bad_operation(x: BadProto) -> None:
    x.f('hello')  # Problematic: doesn't raise a type error.

def some_good_operation(x: GoodProto) -> None:
    x.f('hello')  # Good, raises:
    # [Pyright] Argument of type "Literal['hello']" cannot be assigned to parameter "ell" of type "EllipsisType" in function "f"
    #   "Literal['hello']" is incompatible with "EllipsisType"

The downside of EllipsisType is that it's only available in Python 3.10 onwards. However, we have the following:

assert EllipsisType == type(...)
assert EllipsisType == type(Ellipsis)
assert EllipsisType != ...
assert EllipsisType != Ellipsis
assert ellipsis != Ellipsis  # with ellipsis = TypeVar('ellipsis')

It seems to me like it would make sense to change the current ellipsis = TypeVar('ellipsis') to one of the variants that works. I think it would also be a clarification in the documentation to change ellipsis to EllipsisType and maybe have a note somewhere that those using Python < 3.10 can use the equivalent type(...) or type(Ellipsis).

Potentially related to

• Add an Array Protocol & improve static typing support #589
• Protocol for array objects #229

[rgommers]

Thanks @aaronmondal, that sounds like a useful improvement to me. EllipsisType seems clear enough, it seems unambiguous either way that it means the ... syntax (or the Ellipsis builtin works too, but I've never seen it used in real-world case).

[rgommers]

and maybe have a note somewhere that those using Python < 3.10

A code comment should be fine I think - it doesn't seem desirable to include something like that in the html docs for the standard.

[aaronmondal]

@rgommers Cool, I'll send a PR soon 😊

[BvB93]

Prior to 3.10 you can always use builtins.ellipsis. It's a bit of a semi-private type-check only type (xref python/typeshed#7580), but it's fully functional otherwise and correctly type checks.

EDIT: types.EllipsisType is even defined as an builtins.ellipsis alias in typeshed, so it's not going anywhere any time soon.