ENH: Add Series.set_index

[ghost]

• closes ENH: add Series.set_index #27058, ENH: set_index for Series #21684
• whatsnew entry
• append and list of levels (like DataFrame.set_index has)
• tests added / passed

Help users method-chain all the things. This is modeled closely on the original support for Arrays added in 2011 by Wes Mckinney in

553bacc ENH: can pass external arrays in lieu of column names to DataFrame.set_index

This is also how DataFrame.set_index behaves (though poorly documented), modulo support for column keys which don't apply here.

related discussion #24046, and end of #14829 (comment), #12392

[WillAyd]

I think this has the same issue as the previous PR where the semantics for a Series now break from a DataFrame as the former here accepts list-likes whereas the latter is used for labels.

Haven't kept totally current on this - was this approach ever agreed to in one of the issues or are you just submitting to restart conversation?

[WillAyd]

So to maybe get you up to speed here the semantics of this PR would differ from a DataFrame. Taking your first test case this:

# Sets the index as "A", "B", "C"
>>> pd.Series(range(3)).set_index(["A", "B", "C"])
A    0
B    1
C    2
dtype: int64

Would exhibit different behavior than what happens on a DataFrame with the same number of rows

# Selects columns "A", "B", "C" and moves to the index
>>> pd.DataFrame(np.ones((3,3)), columns=list('ABC')).set_index(["A", "B", "C"])
Empty DataFrame
Columns: []
Index: [(1.0, 1.0, 1.0), (1.0, 1.0, 1.0), (1.0, 1.0, 1.0)]

So in a vacuum this PR would make sense but given the current state of things would need to have something additional to bridge behavior between the two object types.

[pep8speaks]

Hello @pilkibun! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-07-25 04:17:23 UTC

[ghost]

This is very contentious issue among the maintainers it seems, and considering how #22225 ended, seems unlikely this is going anywhere.