GroupBy on a MultiIndex with the index causes python to stop working

[jorisvandenbossche]

If you try following piece of code, my python process crashes:

df = pd.DataFrame({'A':[1,1,2], 'B':[1,2,1], 'C':[1,2,3]})
df = df.set_index(['A', 'B'])
df.index.groupby(df.index)

I know this code is as such probably bullshit (I also did it by accident, the first index shouldn't have been there :-)), but still it should then generate a Error Message instead of causing python to stop?

BTW pd.Index.groupby is a publicly available, but not really clear what is does or is meant for.

[jtratner]

Index methods aren't really public as a rule - they aren't explained in the docs and usually they are just delegated to on certain methods. In the same vein, groupby is only nominally public on Index (as in, it's not really public and you shouldn't be using it).

This crashes because MultiIndex isn't a real ndarray and that method expects to get an ndarray of values.

If you feel strongly this should work, just call _values_from_object in core/common on the object in the groupby method before passing to _groupby.

[jorisvandenbossche]

No, I don't feel strongly at all that this should work, it just shouldn't crash my python process because I mistyped something (it wasn't on purpose that I tried to groupby on the index). And that's why I made an issue about it.

The method is public in the sense that a user sees it when doing tab completion, and that index methods are included in the api docs.

[jreback]

is this groupby actually called internally anywhere? if not should remove it - I believe that this was added a while back when groupby was in flux

maybe convert this issue to provide more validation in groupby itself

[jtratner]

@jreback I think it does get used somewhere -- causes some weird failures about not having the groups property. Index.groubpy is definitely an internal method, but given that it just dispatches to the appropriate algos.pyx method, maybe we could just remove it and have the GroupBy objects call it themselves.

[jtratner]

@jorisvandenbossche this actually works now (probably because of one of the many changes to indexing internals). do you want to close this?

[jreback]

this returns a dict now; it is used 'internally' in groupby.

@jtratner want to do some tests, just proving that this doesn't crash; then can close?

[jorisvandenbossche]

@jtratner ah good you notice it. But before closing, I would:

• or just add a test this does not break again
• or check if it is used somewhere internally and if not remove it? (or if used only a few times, just directly call the underlying function)

[jorisvandenbossche]

@jreback ah, didn't see your comment yet. I will just add some tests then.

[jreback]

@jorisvandenbossche np

Index.groupby is public, meant to be inheritied if an index does need a custom way to do provide groups.

[jorisvandenbossche]

pushed a pr with some tests and a docstring