REGR: FutureWarning issued and empty DataFrame returned where no numeric types to aggregate

• 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 master branch of pandas.

Reproducible Example

import numpy as np
import pandas as pd
frame = pd.DataFrame({"a": np.random.randint(0, 5, 50), "b": ["foo", "bar"] * 25})
frame[["b"]].groupby(frame["a"]).mean()

Issue Description

the code sample was raising DataError: No numeric types to aggregate in 1.2.5 but in 1.3.x this now issues a warning that it will raise in the future. (and also appears that the stacklevel is incorrect)

on master

>>> frame[["b"]].groupby(frame["a"]).mean()
sys:1: FutureWarning: Dropping invalid columns in DataFrameGroupBy.mean is deprecated. In a future version, a TypeError will be raised. Before calling .mean, select only columns which should be valid for the function.
Empty DataFrame
Columns: []
Index: [0, 1, 2, 3, 4]

The change to a empty dataframe was in #41706 with the warning being added in #43154

cc @jbrockmendel @Dr-Irv

Expected Behavior

same as 1.2.5

>>> frame[["b"]].groupby(frame["a"]).mean()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/simon/miniconda3/envs/pandas-1.2.5/lib/python3.9/site-packages/pandas/core/groupby/groupby.py", line 1496, in mean
    return self._cython_agg_general(
  File "/home/simon/miniconda3/envs/pandas-1.2.5/lib/python3.9/site-packages/pandas/core/groupby/generic.py", line 1015, in _cython_agg_general
    agg_mgr = self._cython_agg_blocks(
  File "/home/simon/miniconda3/envs/pandas-1.2.5/lib/python3.9/site-packages/pandas/core/groupby/generic.py", line 1121, in _cython_agg_blocks
    raise DataError("No numeric types to aggregate")
pandas.core.base.DataError: No numeric types to aggregate

Installed Versions

pd.show_versions()

[Dr-Irv]

@rshadrach so I’ve taken the table I made above and added a “Proposal” column, which I think is what you are suggesting above.

Code	1.2.5 Result	1.3.2 Result	1.4.0.dev0+647.g0072fa8cb5 Result	Proposal
frame[["b"]].groupby(frame["a"]).mean()	DataError exception	Empty DataFrame	FutureWarning and Empty DataFrame	Empty DataFrame
frame[["b"]].groupby(frame["a"]).agg("mean")	DataError exception	Empty DataFrame	FutureWarning and Empty DataFrame	Empty DataFrame
frame[["b"]].groupby(frame["a"]).apply(np.mean)	Empty DataFrame	FutureWarning and Empty DataFrame	FutureWarning and Empty DataFrame	Empty DataFrame
frame[["b"]].groupby(frame["a"]).sum()	DataFrame with values	Empty DataFrame	DataFrame with values	DataFrame with values
frame[["b"]].groupby(frame["a"]).agg("sum")	DataFrame with values	Empty DataFrame	DataFrame with values	DataFrame with values
frame[["b"]].groupby(frame["a"]).apply(np.sum)	DataFrame with values	DataFrame with values	DataFrame with values	DataFrame with values

That would replicate the 1.2.5 behavior except where we were raising the DataError, replacing that with an empty DataFrame. It also seems that this would be the easiest fix to make??

[simonjayhawkins]

sure. closing as no action.