BUG: Unexpected KeyError in `transform()` with dict-like func argument

[sfo]

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Reproducible Example

import pandas as pd

pd.DataFrame({'foo': [2,4,6], 'bar': [1,2,3]}).transform(
    {
        'foo': lambda x: x+2,
        'bar': lambda x: x*2
    },
    axis="columns"
)

Issue Description

The code raises KeyError: "Column(s) ['bar', 'foo'] do not exist" even though the columns are present, evidently.

Expected Behavior

Transform the columns according to the dict-like argument of the tranform() function and return a new data frame, without raising an error.

Installed Versions

INSTALLED VERSIONS

commit : d9cdd2ee5a58015ef6f4d15c7226110c9aab8140 python : 3.11.9.final.0 python-bits : 64 OS : Linux OS-release : 6.8.7-200.fc39.x86_64 Version : #1 SMP PREEMPT_DYNAMIC Wed Apr 17 19:35:11 UTC 2024 machine : x86_64 processor : byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8

pandas : 2.2.2 numpy : 1.24.2 pytz : 2022.7.1 dateutil : 2.8.2 setuptools : 67.1.0 pip : 24.0 Cython : None pytest : None hypothesis : None sphinx : None blosc : None feather : None xlsxwriter : None lxml.etree : 4.9.2 html5lib : None pymysql : None psycopg2 : None jinja2 : 3.1.2 IPython : 8.10.0 pandas_datareader : None adbc-driver-postgresql: None adbc-driver-sqlite : None bs4 : 4.11.2 bottleneck : None dataframe-api-compat : None fastparquet : 2023.2.0 fsspec : 2023.1.0 gcsfs : None matplotlib : 3.6.3 numba : None numexpr : None odfpy : None openpyxl : 3.1.1 pandas_gbq : None pyarrow : 16.0.0 pyreadstat : None python-calamine : None pyxlsb : None s3fs : None scipy : 1.10.0 sqlalchemy : 2.0.24 tables : None tabulate : None xarray : 2023.2.0 xlrd : 2.0.1 zstandard : None tzdata : 2024.1 qtpy : 2.3.0 pyqt5 : None

[Aloqeely]

You can pass axis="index" to apply the function to each column (I'm not quite sure what that means) per the doc at https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.transform.html

But I will admit this is a little confusing, I don't know why the error message says Column(s) [...] do not exist, the error message should be improved. And I also find the docs to be a little inconsistent when it comes to the axis parameter, some docs say axis="index" will compute row-wise, and some say apply on each column, the only issue related issue I could found is #43128

[rhshadrach]

But I will admit this is a little confusing, I don't know why the error message says Column(s) [...] do not exist, the error message should be improved.

Agreed.

And I also find the docs to be a little inconsistent when it comes to the axis parameter, some docs say axis="index" will compute row-wise, and some say apply on each column, the only issue related issue I could found is #43128

Best to see the context, but I believe row-wise is meant as "down the rows", which is the same as applying to each column.

[luke396]

take