DOC: Add documentation for groupby.expanding()

[olek-osikowicz]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

"https://pandas.pydata.org/docs/dev/reference/groupby.html"

Documentation problem

There is no reference for DataFrameGroupBy.expanding(), even though it exists in API

E.g. consider working example:

>>> import pandas as pd
>>> pd.__version__
'2.2.3'
>>> data = {"Class": ["A", "A", "A", "B", "B", "B"],"Value": [10, 20, 30, 40, 50, 60],}
>>> df = pd.DataFrame(data)
>>> df
  Class  Value
0     A     10
1     A     20
2     A     30
3     B     40
4     B     50
5     B     60
>>> expanding_mean = df.groupby("Class").expanding().mean().reset_index(drop=True)
>>> expanding_mean
   Value
0   10.0
1   15.0
2   20.0
3   40.0
4   45.0
5   50.0

It's undocumented behaviour

Suggested fix for documentation

Include reference of DataFrameGroupBy.expanding and SeriesGroupBy.expanding, like for DataFrameGroupBy.rolling
and SeriesGroupBy.rolling

[rhshadrach]

Thanks for the report. Agreed this should be added. While adding it to the API docs will make it show up, it looks like the current docstring should also be greatly improved as well.

PRs to improve are welcome!

[arthurlw]

take