Description
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 master branch of pandas.
Reproducible Example
pd.DataFrame.from_dict({
'A': [1, 2],
'B': [3, 3],
'C': ['G', 'G'],
}).groupby('C')[['A', 'B', 'A']].mean()Issue Description
The above fails with an error like this:
File "C:\Users\mboling\Anaconda3\envs\pandastest2\lib\site-packages\pandas\core\groupby\generic.py", line 1538, in __getitem__
return super().__getitem__(key)
File "C:\Users\mboling\Anaconda3\envs\pandastest2\lib\site-packages\pandas\core\base.py", line 222, in __getitem__
raise KeyError(f"Columns not found: {str(bad_keys)[1:-1]}")
KeyError: 'Columns not found: '
This is misleading -- the A and B columns are all present in the input, the real problem is that the A column is repeated.
Expected Behavior
It would be better to either not fail in this case (i.e. return a DataFrame with duplicate column names) or fail with an error message that mentions that the input columns contain duplicates.
Installed Versions
Tested on Pandas 1.3.4, but that install has a broken show_versions for some reason, so this is show_versions from an older install that also exhibits the problem.
pandas : 1.3.4
numpy : 1.21.3
pytz : 2021.3
dateutil : 2.8.2
pip : 21.1.1
setuptools : 58.0.4
Cython : 0.29.24
pytest : 6.2.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 3.0.2
IPython : 7.29.0
pandas_datareader: None
bs4 : 4.10.0
bottleneck : 1.3.2
fsspec : 2021.08.1
fastparquet : None
gcsfs : None
matplotlib : 3.4.3
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.9
pandas_gbq : None
pyarrow : 3.0.0
pyxlsb : None
s3fs : None
scipy : 1.6.2
sqlalchemy : None
tables : 3.6.1
tabulate : None
xarray : 0.19.0
xlrd : 2.0.1
xlwt : None
numba : 0.53.1