DOC: No warning in set_index() that previous index column is removed.

[ncotie]

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/reference/api/pandas.DataFrame.set_index.html

Documentation problem

set_index(), when applied to a DataFrame which already has a data column (non-default) assigned as index, will delete this data column from the DataFrame when assigning another data column to be the index.

While I find this behaviour inappropriate, I understand that reset_index() should be used before set_index(), in which case the original index column may be preserved.

The problem is that the documentation for set_index() does not mention this at all, so the user is left to discover the problem and then the way to avoid it.

Suggested fix for documentation

Add a comment in the set_index documentation to clarify that setting a data column as index, when there is already a different data column serving as index, will delete that data column, unless reset_index is performed first.

[SaraInCode]

take

[rhshadrach]

Thanks for the report!

The problem is that the documentation for set_index() does not mention this at all, so the user is left to discover the problem and then the way to avoid it.

The documentation states:

The index can replace the existing index or expand on it.

Doesn't this count as a mention?

[ncotie]

Well, in my reading, 'replacing the existing index' means substituting it in its function as index, but isn't the same as 'causes the existing index data to be deleted entirely'.

What I'm getting at is that I (and I suppose I'm not alone in this) wouldn't see it as normal that changing the role of index from one data column to another would delete data. Taken to the extreme, one could delete the DataFrame entirely by changing the index column from one to the next, until there were none left.