BUG: transform("max") replaces all entries with NaN when applied after groupby with as_index=False · Issue #49834 · pandas-dev/pandas (original) (raw)

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Reproducible Example

import pandas as pd

df = pd.DataFrame({"key": ["a", "b", "c", "b", "a"], "value": [False, False, True, False, True]}, index=[7, 2, 5, 3, 9])

df

Out[4]: key value 7 a False 2 b False 5 c True 3 b False 9 a True

df.groupby("key", as_index=False).transform("max")

Out[5]: key value 7 NaN NaN 2 NaN NaN 5 NaN NaN 3 NaN NaN 9 NaN NaN

df.groupby("key", as_index=True).transform("max")

Out[6]: value 7 True 2 False 5 True 3 False 9 True

Issue Description

When using transform("max") after groupby with as_index=False, all entries in the original dataframe are converted to NaN. This is not only the case for "max" but also for "min" as well as "sum" or "mean" when having integers in the "value" column.

When using as_index=True instead, the transform works as expected, however, the "key" column does not appear as an index but simply disappears from the dataframe.

Expected Behavior

1. I would expect that when using as_index=False only the entries in the "value" column change, i.e. change to

True
False
True
False
True

1. I would expect that when using as_index=True the "key" column appears as an index.

Installed Versions

INSTALLED VERSIONS

commit : 91111fd
python : 3.8.10.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.104-linuxkit
Version : #1 SMP Thu Mar 17 17:08:06 UTC 2022
machine : x86_64
processor :
byteorder : little
LC_ALL : C.UTF-8
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.5.1
numpy : 1.23.4
pytz : 2022.4
dateutil : 2.8.2
setuptools : 58.1.0
pip : 21.2.4
Cython : None
pytest : 7.2.0
hypothesis : 5.23.7
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.8.6
jinja2 : 3.1.2
IPython : 8.5.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : None
brotli : None
fastparquet : None
fsspec : 2022.02.0
gcsfs : None
matplotlib : 3.5.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 5.0.0.2
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.9.0
snappy : None
sqlalchemy : 1.3.24
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : 0.14.0
tzdata : None