BUG: Update FutureWarning message to be more clear

[RyanBerger98]

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({"ID": [1,1,2,2,3],"B": [1,10,5,5,3]})
df_new = df[df.groupby(["ID"])["B"].transform(max) == df["B"]]

Issue Description

This isn't necessarily a bug, but the other issue categories didn't fit it either.

In #53974, the use of built-in callable functions for SeriesGroupBy.transform()was deprecated. The current FutureWarning message that is displayed is:

E       FutureWarning: The provided callable <built-in function max> is currently using SeriesGroupBy.max. In a future version of pandas, the provided callable will be used directly. To keep current behavior pass 'max' instead.
/pandas/core/apply.py:1828: FutureWarning

While the change is reasonable, I think the warning message could be updated to better convey the message. When I first saw the warning message, I didn't realize what was wrong. Obviously using the built-in functions was no longer supported, but the message said to keep current behavior pass 'max' instead, however this is exactly what I was doing, but I was passing the callable max instead.

After some time of reading the release notes and documentation, I realized the warning message was referring to the string "max".

Expected Behavior

My proposal is to just update the FutureWarning produced from this to explicitly state a string should be passed. Updating

FutureWarning: .... To keep current behavior pass 'max' instead.

to

FutureWarning: .... To keep current behavior pass the string 'max' instead.

Installed Versions

2.1.0

[lithomas1]

Thanks for opening this issue.

A PR would be welcome for this.

[rsm-23]

@RyanBerger98 do you wish to work on this? Otherwise I'll take it up.

[rhshadrach]

Perhaps

pass the string `"max"` instead

would be even more clear.

[rsm-23]

take

[RyanBerger98]

@rsm-23 thanks for picking this up!