s.str.upper doesn't preserve s type

[MarcoGorelli]

import pandas as pd

s: "pd.Series[str]" = pd.Series(['a', 'b'])
reveal_type(s)  # information: Type of "s" is "Series[str]"
reveal_type(s.str.upper())  # information: Type of "s.str.upper()" is "Series[Unknown]"

The reason I'm reporting this is that the source code uses a TypeVar, so the intention might have been that the type be preserved?

pandas-stubs/pandas-stubs/core/strings.pyi

Line 180 in d410f2e

def upper(self) -> T: ...

[loicdiridollou]

The issue here is less about the T but I think is closer to the str property which does not preserve the type of s.
If you try to highlight the type of s.str you realize that it has lost the information about whether it is a Series[str] or anything else.
I can’t remember how to properly type hint properties but that may be an idea.

[Dr-Irv]

I think that the declaration of str in Series should add an argument to StringMethods related to the subtype of Index or Series, i.e., include S1 in that signature so that the bound value of S1 gets passed down. At least that's my hunch!