BUG: `DataFrameGroupBy.apply` does not take into account the value of `as_index` field

[anmyachev]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import numpy as np

df = pd.DataFrame(
    {
        "col1": [0, 1, 2, 3],
        "col2": [4, 5, np.NaN, 7],
        "col3": [np.NaN, np.NaN, 12, 10],
        "col4": [17, 13, 16, 15],
        "col5": [-4, -5, -6, -7],
    }
)

print(df.groupby(by=[1,2,1,2], as_index=False).apply(lambda df: df.sum()))
print(df.groupby(by=[1,2,1,2], as_index=True).apply(lambda df: df.sum()))

Issue Description

as_index parameter does not affect the result of apply.

Expected Behavior

>>> pandas_df.groupby(by=[1,2,1,2], as_index=True).apply(lambda df: df.sum())
   col1  col2  col3  col4  col5
1   2.0   4.0  12.0  33.0 -10.0
2   4.0  12.0  10.0  28.0 -12.0

>>> pandas_df.groupby(by=[1,2,1,2], as_index=False).apply(lambda df: df.sum())   # <- expected output
   index  col1  col2  col3  col4  col5
0      1   2.0   4.0  12.0  33.0 -10.0
1      2   4.0  12.0  10.0  28.0 -12.0

Installed Versions

pandas 2.0.2
-- Replace this line with the output of pd.show_versions()

[rhshadrach]

Thanks for the report! Due to #52333 the following warning is now emitted on the as_index=False line:

FutureWarning: A grouping was used that is not in the columns of the DataFrame and so was excluded from the result. This grouping will be included in a future version of pandas. Add the grouping as a column of the DataFrame to silence this warning.

This deprecation will appear in 2.1. Once the deprecation is enforced in pandas 3.0, this will be fixed.

[anmyachev]

Thanks for the report! Due to #52333 the following warning is now emitted on the as_index=False line:

FutureWarning: A grouping was used that is not in the columns of the DataFrame and so was excluded from the result. This grouping will be included in a future version of pandas. Add the grouping as a column of the DataFrame to silence this warning.

This deprecation will appear in 2.1. Once the deprecation is enforced in pandas 3.0, this will be fixed.

@rhshadrach thanks for the answer!