BUG: DataFrame.pivot(index=None) raises KeyError

[douglas-raillard-arm]

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Reproducible Example

import pandas as pd

df = pd.DataFrame(dict(a=[0,1,1,0], b=[42,43,44,45]), index=[10,20,30,40])
pivoted = df.pivot(index=None, columns='a', values='b')
print(pivoted)

Issue Description

DataFrame.pivot() documentation states:

index: str or object or a list of str, optional

    Column to use to make new frame’s index. If None, uses existing index.

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.pivot.html

In pandas 1.5, index=None provides the expected result. In pandas 2.0.0, it raises a KeyError exception.

Expected Behavior

index=None should use the existing index as per documentation, or documentation should be modified and 2.0.0 breaking changes list updated to add that new behavior.

Installed Versions

INSTALLED VERSIONS ------------------ commit : 478d340 python : 3.8.10.final.0 python-bits : 64 OS : Linux OS-release : 5.15.0-60-generic Version : #66~20.04.1-Ubuntu SMP Wed Jan 25 09:41:30 UTC 2023 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8

pandas : 2.0.0
numpy : 1.24.2
pytz : 2023.3
dateutil : 2.8.2
setuptools : 67.6.1
pip : 23.0.1
Cython : None
pytest : 7.2.2
hypothesis : None
sphinx : 6.1.3
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.9.2
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.12.0
pandas_datareader: None
bs4 : 4.12.0
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.7.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 11.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.10.1
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[MarcoGorelli]

Thanks @douglas-raillard-arm for the report

This was caused by #48293 cc @phofl

https://www.kaggle.com/code/marcogorelli/pandas-regression-example?scriptVersionId=124667072

[MarcoGorelli]

Looks like

df.pivot(columns='a', values='b')

gives you your expected output, so maybe the docs just need to be updated to say "Column to use to make new frame’s index. If not given, uses existing index."

[phofl]

I‘d say that this behaves as intended and needs a doc update