Return type from pandas.core.groupby.generic.DataFrameGroupBy.resample

[vectro]

Describe the bug
The type hint for pandas.core.groupby.generic.DataFrameGroupBy.resample says it returns a pandas.core.groupby.grouper.Grouper; however in at least some situations this function can return a pandas.core.resample.DatetimeIndexResamplerGroupby, which is not a Grouper.

To Reproduce
This results in a false positive on MyPy 1.7.1:

df.set_index("date").groupby("intensity").resample("90D")["duration"]

The error message is

error: Value of type "Grouper" is not indexable  [index]

Please complete the following information:
Python 3.11
Mypy 1.7.1
pandas-stubs 2.1.1.230928

[Dr-Irv]

Thanks for the report. Looking at the docs, we should change

to return a union of pandas.api.typing.DatetimeIndexResamplerGroupby,
pandas.api.typing.PeriodIndexResamplerGroupby, and
pandas.api.typing.TimedeltaIndexResamplerGroupby

Unfortunately, we haven't added the types for pandas.api.typing yet, nor do we have DatetimeIndexResamplerGroupby in core/resample.pyi . So that latter type should get added there, and can be the return type of resample(), since the other 2 types are subclasses of the first one.

Some care will need to be exercised, because in the stubs we are doing some things with respect to generic types that will have to be appropriately passed down for things to work.