KeyError when retrieving columns with lists of incomplete tuples from MultiIndex

[toobaz]

Sorry in advance if this was already reported - I searched a bit but didn't know exactly what to search.

In [2]: df = pd.DataFrame(0, index=range(1), columns=pd.MultiIndex.from_product([['a', 'b'], ['c']]))

In [3]: cols = [('a',), ('b',)]

In [4]: df[cols[0]]
Out[4]: 
   c
0  0

In [5]: df[cols[1]]
Out[5]: 
   c
0  0

In [6]: df[cols]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-6-957cf8a677a9> in <module>()
----> 1 df[cols]

/home/pietro/nobackup/repo/pandas/pandas/core/frame.pyc in __getitem__(self, key)
   1975         if isinstance(key, (Series, np.ndarray, Index, list)):
   1976             # either boolean or fancy integer index
-> 1977             return self._getitem_array(key)
   1978         elif isinstance(key, DataFrame):
   1979             return self._getitem_frame(key)

/home/pietro/nobackup/repo/pandas/pandas/core/frame.pyc in _getitem_array(self, key)
   2019             return self.take(indexer, axis=0, convert=False)
   2020         else:
-> 2021             indexer = self.ix._convert_to_indexer(key, axis=1)
   2022             return self.take(indexer, axis=1, convert=True)
   2023 

/home/pietro/nobackup/repo/pandas/pandas/core/indexing.pyc in _convert_to_indexer(self, obj, axis, is_setter)
   1203                 mask = check == -1
   1204                 if mask.any():
-> 1205                     raise KeyError('%s not in index' % objarr[mask])
   1206 
   1207                 return _values_from_object(indexer)

KeyError: "[('a',) ('b',)] not in index"

Notice that df[[c[0] for c in cols]] works just fine... but still we probably want to consider as valid a list of valid keys.

[jreback]

you have to use multiindex slicers if you want partial selections

In [12]: df.loc[:,['a','b']]
Out[12]: 
   a  b
   c  c
0  0  0

[] is too ambiguous for things like this

[toobaz]

(I just thought the above would be expected to work fine, but df[['a', 'b']] actually does the job too)

[toobaz]

So this neither is a bug, right?

In [2]: df = pd.DataFrame(index=range(2), columns=pd.MultiIndex.from_product([[10,20], ['a', 'b']]))

In [3]: df[[20]]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-3-8422ebb3f356> in <module>()
----> 1 df[[20]]

/home/pietro/nobackup/repo/pandas/pandas/core/frame.pyc in __getitem__(self, key)
   1975         if isinstance(key, (Series, np.ndarray, Index, list)):
   1976             # either boolean or fancy integer index
-> 1977             return self._getitem_array(key)
   1978         elif isinstance(key, DataFrame):
   1979             return self._getitem_frame(key)

/home/pietro/nobackup/repo/pandas/pandas/core/frame.pyc in _getitem_array(self, key)
   2020         else:
   2021             indexer = self.ix._convert_to_indexer(key, axis=1)
-> 2022             return self.take(indexer, axis=1, convert=True)
   2023 
   2024     def _getitem_multilevel(self, key):

/home/pietro/nobackup/repo/pandas/pandas/core/generic.pyc in take(self, indices, axis, convert, is_copy)
   1590         new_data = self._data.take(indices,
   1591                                    axis=self._get_block_manager_axis(axis),
-> 1592                                    convert=True, verify=True)
   1593         result = self._constructor(new_data).__finalize__(self)
   1594 

/home/pietro/nobackup/repo/pandas/pandas/core/internals.pyc in take(self, indexer, axis, verify, convert)
   3617         n = self.shape[axis]
   3618         if convert:
-> 3619             indexer = maybe_convert_indices(indexer, n)
   3620 
   3621         if verify:

/home/pietro/nobackup/repo/pandas/pandas/core/indexing.pyc in maybe_convert_indices(indices, n)
   1803     mask = (indices >= n) | (indices < 0)
   1804     if mask.any():
-> 1805         raise IndexError("indices are out-of-bounds")
   1806     return indices
   1807 

IndexError: indices are out-of-bounds

(it is interpreting the "20" as index rather than label)
For instance the following "works" instead:

In [4]: df[[1]]
Out[4]: 
    10
     b
0  NaN
1  NaN

[jreback]

they both should prob raise KeyError

might be non trivial to fix