Discussion: Should reindex keep the index name if new index has none

[hsperr]

As mentioned in #6552 there is an open question about whether reindex should keep the index name if the new index has no name.

Current Behavior:

• reindex keeps index name if the argument is a list or an ndarray
• replaces the index name if the argument is an index, even if the new index does not have a new name.

Related Issues:
#9854

[TomAugspurger]

There isn't an obvious answer to me for this one.

Also keep in mind that we have reindex_like. That one isn't clear either, but they should probably have the same behavior? Maybe?

[shoyer]

I think reindex and reindex_like should definitely have the same behavior.

We do technically associate the name of an index with the index itself, but from a practical perspective the name of an index is also strongly associated with the dataframe that contains it (somewhat similarly to the case for Series.name and DataFrame.columns). In my opinion, it is a little surprising (even if it happens rarely) for reindexing to rename existing indexes -- it makes it harder to understand how the data was transformed. So my vote is for reindexing to never change index names.

[attack68]

I'm developing an application which uses this feature. I came across page researching a bug in my application. I have used re_index to readjust on the fly to a function local Index variable which I did not bother to assign a name to. Reindex therefore dropped my index name and it broke down the line.

I would suggest that the index name is preserved if:

the reindex argument is a list, ndarray or Index which does not have a name,

and the index name is renamed if:

the reindex argument is an Index which has a specific name.

Perhaps an optional 'name_transform=None' argument could be added with the default behaviour as above and further 'drop' or 'keep' options to explicitly define the behaviour.

[mroeschke]

Thanks for the issue but it appears there hasn't been much movement towards an action item here so closing. Happy to have a new issue with the discussion if there's interest