BUG: axis=None when given as a parameter of skew behaves differently when used in combination with groupy

[marenwestermann]

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

df = pd.DataFrame({'a': [1, 1, 1, 1], 'b': [2, 3, 4, 20], 'c': [3, 4, 5, -20]})
print(df[['b', 'c']].skew(axis=None))
# -0.9424450936999188

gb = df.groupby('a')
result = gb.skew(axis=None)
print(result)
#           b         c
# a                    
# 1  1.945331 -1.972398

Issue Description

In the documentation of Series.skew it is said about the axis parameter:
"For DataFrames, specifying axis=None will apply the aggregation across both axes."
However, this does not seem to work when a groupby is applied to a dataframe as shown in the example above.

This issue was discovered and the code snippet was provided in #50958

Expected Behavior

import pandas as pd

df = pd.DataFrame({'a': [1, 1, 1, 1], 'b': [2, 3, 4, 20], 'c': [3, 4, 5, -20]})
print(df[['b', 'c']].skew(axis=None))
# -0.9424450936999188

gb = df.groupby('a')
result = gb.skew(axis=None)
print(result)
# -0.9424450936999188

Installed Versions

INSTALLED VERSIONS
------------------
commit           : 7ace24e8adc7ae03ce9acdbcadaf3ef42a6f2960
python           : 3.8.15.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 22.2.0
Version          : Darwin Kernel Version 22.2.0: Fri Nov 11 02:03:51 PST 2022; root:xnu-8792.61.2~4/RELEASE_ARM64_T6000
machine          : arm64
processor        : arm
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 2.0.0.dev0+1304.g7ace24e8ad
numpy            : 1.23.5
pytz             : 2022.7
dateutil         : 2.8.2
setuptools       : 65.6.3
pip              : 22.3.1
Cython           : 0.29.32
pytest           : 7.2.0
hypothesis       : 6.62.0
sphinx           : 4.5.0
blosc            : None
feather          : None
xlsxwriter       : 3.0.6
lxml.etree       : 4.9.2
html5lib         : 1.1
pymysql          : 1.0.2
psycopg2         : 2.9.3
jinja2           : 3.1.2
IPython          : 8.8.0
pandas_datareader: None
bs4              : 4.11.1
bottleneck       : 1.3.5
brotli           :
fastparquet      : 2022.12.0
fsspec           : 2022.11.0
gcsfs            : 2022.11.0
matplotlib       : 3.6.2
numba            : 0.56.4
numexpr          : 2.8.3
odfpy            : None
openpyxl         : 3.0.10
pandas_gbq       : None
pyarrow          : 10.0.1
pyreadstat       : 1.2.0
pyxlsb           : 1.0.10
s3fs             : 2022.11.0
scipy            : 1.10.0
snappy           :
sqlalchemy       : 1.4.45
tables           : 3.7.0
tabulate         : 0.9.0
xarray           : 2022.12.0
xlrd             : 2.0.1
zstandard        : 0.19.0
tzdata           : None
qtpy             : None
pyqt5            : None

[rhshadrach]

Thanks for raising this issue - I think the expected result of gb.skew(axis=None) is a Series, one row for each group.

Another option is just to remove None from the axis argument in groupby (but keeping it in DataFrame.skew)