API: select levels of a MultiIndex

[jorisvandenbossche]

Say you have a multi-index:

In [34]: idx = pd.MultiIndex.from_product([['a', 'b', 'c'], [1, 2, 3], ['f', 'g'
]], names=['lev0', 'lev1', 'lev2'])

In [35]: df = pd.DataFrame(range(len(idx)), index=idx)

In [36]: df
Out[36]:
                 0
lev0 lev1 lev2
a    1    f      0
          g      1
     2    f      2
          g      3
     3    f      4
          g      5
b    1    f      6
          g      7
     2    f      8
          g      9
     3    f     10
          g     11
c    1    f     12
          g     13
     2    f     14
          g     15
     3    f     16
          g     17

and you want to select certain levels of the Index (like you select columns of a frame, I want to select levels of an index and get a subset of the index).

At the moment, some possibilities:

In [37]: pd.MultiIndex.from_arrays([df.index.get_level_values(0), df.index.get_level_values(1)])
Out[37]:
MultiIndex(levels=[[u'a', u'b', u'c'], [1, 2, 3]],
           labels=[[0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2], [0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2]],
           names=[u'lev0', u'lev1'])

In [38]: idx.droplevel(-1)    # if you know the ones to drop
Out[38]:
MultiIndex(levels=[[u'a', u'b', u'c'], [1, 2, 3]],
           labels=[[0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2], [0, 0
, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2]],
           names=[u'lev0', u'lev1'])

In [39]: df.reset_index().set_index(['lev0','lev1']).index
Out[39]:
MultiIndex(levels=[[u'a', u'b', u'c'], [1, 2, 3]],
           labels=[[0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2], [0, 0
, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2]],
           names=[u'lev0', u'lev1'])

In [40]: df.reset_index(-1).index       # if you know the ones to drop
Out[40]:
MultiIndex(levels=[[u'a', u'b', u'c'], [1, 2, 3]],
           labels=[[0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2], [0, 0
, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2]],
           names=[u'lev0', u'lev1'])

Am I missing an easy way to do this?

And if not, I think we should have a better way to do this.

Note: triggerd by this SO question: http://stackoverflow.com/questions/31991388/combinations-of-multiindex-levels-which-occur-in-a-dataframe (but had already encountered this multiple times)

[rockg]

I swear that there was an old issue/PR that addressed this. If I recall it was proposed do have syntax like df.index['lev0'] or perhaps even df['lev0'] both of which I think are much better than get_level_values which is too verbose. However, searches have come up empty. #10461 seems to discuss some variants.

[jorisvandenbossche]

There is the proposal that the index, if it has a name, should also be accessible as a column (eg df['lev0']), but also returning a series then. See #8162, and although I think that would be very nice, this is still something else.

[nehalecky]

+1

[jorisvandenbossche]

Somebody a good idea for a possible API here?

Some ideas (selecting a single level here as example, but it should expand to list of names as well) :

• df.index["level_name"] -> as index __getitem__ is already used to select values of the index, I don't think it would be a good design to also let this select levels of the index (and, it would conflict in case of integer level names)
• df.index.levels["level_name"] -> this is already used to select the actual 'level' (so the unique values, or the categories using that terminology) and not for the full level values. So again, that it does something different with a string would not be good design (and it would conflict with integer level names)
• df.index.get_levels("level_name") -> this I would like (and doesn't exist yet), but, it would not be consistent with what set_levels does (it would not be its opposite, what you would expect from the name), as set_levels sets actual 'levels' and not level values.
• df.index.get_level_values(["level_name", ..]) -> expand the existing get_level_values to also accept a list of integers/names instead of only a scalar, and in that case returning a MultiIndex instead of Index. This is probably the easiest to add.

[jreback]

why would you do this, rather than #8162?

[jorisvandenbossche]

In my mind, this is something different, as I said above (#10816 (comment)). But of course that is something to discuss in #8162, but in my mind accessing an index like a column, should also return it as a column, and here I want to select specific levels but still keep an index as the final result.

[jreback]

My question is why would you want to add moar API? what is the usecase?

df.index.get_level_values(["level_name", ..]) this seems reasonable actually (though a single level and a list of a single level would be the same?, or would first be a Series, 2nd DataFrame)?