BUG: pd.Series.mode failed to give expected result when it is embedded inside groupby.transform

[Fanghaozhi-Liang]

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• 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

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
                          'foo', 'bar'],
                   'B' : ['one', 'one', 'two', 'three',
                          'two', 'two'],
                   'C' : [1, 5, 5, 2, 5, 5],
                   'D' : [2.0, 5., 8., 1., 2., 9.]})

df.groupby(['A'])['C'].transform(pd.Series.mode)

Issue Description

The result includes unexpected NA value.

Expected Behavior

import statistics
import pandas as pd

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
'foo', 'bar'],
'B' : ['one', 'one', 'two', 'three',
'two', 'two'],
'C' : [1, 5, 5, 2, 5, 5],
'D' : [2.0, 5., 8., 1., 2., 9.]})

df.groupby(['A'])['C'].transform(statistics.mode)

Installed Versions

INSTALLED VERSIONS

commit : 4bfe3d0
python : 3.8.8.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.22000
machine : AMD64
processor : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : Chinese (Simplified)_China.1252

pandas : 1.4.2
numpy : 1.21.5
pytz : 2021.1
dateutil : 2.8.1
pip : 22.1.1
setuptools : 62.1.0
Cython : 0.29.23
pytest : 6.2.3
hypothesis : None
...
xarray : 0.21.1
xlrd : 2.0.1
xlwt : 1.3.0
zstandard : None

[simonjayhawkins]

Thanks @Fanghaozhi-Liang for the report.

Does statistics.mode give the expected result when then is more that one mode?

pd.Series.mode returns a Series of the modes of the Series in sorted order and zero-indexed by that sort order

groupby.transform expects a call function producing a like-indexed Series on each group and return a Series having the same indexes as the original object filled with the transformed values.

pd.Series.mode does not fulfil the requirements of a grouby.transform callable.

for _, ser in df.groupby(["A"])["C"]:
    print(pd.Series.mode(ser))

0    5
Name: C, dtype: int64
0    5
Name: C, dtype: int64

[phofl]

Closing for now. Feel free to ping to reopen, if you think this is a bug on the pandas side