ENH: Index.method(target: Index[same_dtype_as_self])

[jbrockmendel]

There are a bunch of Index methods e.g. _get_indexer, _union that take an other or target argument that should be another Index with the same dtype as self. I'd like a way to express that via annotation.

[loicdiridollou]

I think the Self annotation from the typing module should suffice. You may encounter a glitch with index, for example if you do Index._union(DatetimeIndex) won't raise an issue as DatetimeIndex is a subclass of Index. But if you check PeriodIndex._union(DatetimeIndex) in that case the typechecker will complain. I don't believe there is a way to check for Index._union(DatetimeIndex) since one is a subclass of the other, @Dr-Irv can you think of another way?
Does this solve your issue?

[jbrockmendel]

That doesnt distinguish between Index[np.float64] vs Index[np.int64], similar for different dtypes that can be held by a DatetimeIndex.

[loicdiridollou]

In the stubs we don't really use the numpy types, but could you try this:

from typing import TypeVar

IndexT = TypeVar("IndexT", bound="Index")

class Index:
    def _union(self: IndexT, other: IndexT, sort: bool | None) -> TypeYouWant: ...

The only issue is that if the type of the index inside is a subclass of the index outside or inversely (like Index[Any] and Index[int]) then it would not complain but it will for Index[int] and Index[float].
Let me know if that helps

[Dr-Irv]

I think the Self annotation from the typing module should suffice. You may encounter a glitch with index, for example if you do Index._union(DatetimeIndex) won't raise an issue as DatetimeIndex is a subclass of Index. But if you check PeriodIndex._union(DatetimeIndex) in that case the typechecker will complain. I don't believe there is a way to check for Index._union(DatetimeIndex) since one is a subclass of the other, @Dr-Irv can you think of another way?

Not really, but it depends on what we want to do. Leaving the generic aspect of Index aside for the moment, if we consider the class Index and subtypes DatetimeIndex, TimedeltaIndex, RangeIndex, etc. and we just consider the union method, what we have now will disallow DatetimeIndex.union(TimedeltaIndex), for example. But because Index is the super class, I don't think we can prevent DatetimeIndex.union(Index) or Index.union(DatetimeIndex), because of the Liskov substitution principle.

There's a possibility that creating a TypeVar to represent the valid types that could be passed would solve the problem.