DOC: Is the behaviour of setitem with repeated keys defined?

[wence-]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/docs/user_guide/indexing.html

(and loc/iloc docstrings)

Documentation problem

Suppose that I do this:

import pandas as pd

s = pd.Series([1, 2, 3, 4])

s.iloc[[0, 0]] = [7, 8]
# Or, similarly with `loc`

This is in some sense a nonsense thing to do, but is it therefore "undefined behaviour" (in the standardese sense) and pandas might do anything. Or is it defined that "last index wins"? Or is it an implementation detail?

Suggested fix for documentation

Clarifying what is/is not allowed when setting values would be useful. My guess here is that the fact that the last value wins out is an implementation detail which probably should not be relied on.

[dbalagula]

Take