groupby(...).transform is untyped

[rhshadrach]

Describe the bug
SeriesGroupBy.transform and DataFrameGroupBy.transform are currently not typed:

pandas-stubs/pandas-stubs/core/groupby/generic.pyi

Line 159 in 001c0af

def transform(self, func, *args, **kwargs): ...

Running mypy with --disallow-untyped-calls reports an issue with any usage of this method.

[Dr-Irv]

Need to return Series for SeriesGroupBy.transform and DataFrame for DataFrameGroupBy.transform.

PR with tests welcome

[ramvikrams]

Do we not have to change the arguments of transform? will it work if we just add the return types

[Dr-Irv]

Do we not have to change the arguments of transform? will it work if we just add the return types

For now, you could just annotate f as Callable and don't worry about *args and **kwargs, and then add the return types.

[rhshadrach]

For now, you could just annotate f as Callable

I would do str | Callable; using a string alias is a very common use case (at least for me). Looks like the docs need updating in this regard.