---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-28-547c2aa43ab1> in <module>()
     11 new_df = df.copy()
     12
---> 13 new_df['size'] = new_df.groupby(['A','B']).transform(np.size)
     14
     15 new_df

C:\Python\Python32\lib\site-packages\pandas\core\frame.pyc in __setitem__(self, key, value)
   2036         else:
   2037             # set column
-> 2038             self._set_item(key, value)
   2039
   2040     def _setitem_slice(self, key, value):

C:\Python\Python32\lib\site-packages\pandas\core\frame.pyc in _set_item(self, key, value)
   2083         """
   2084         value = self._sanitize_column(key, value)
-> 2085         NDFrame._set_item(self, key, value)
   2086
   2087     def insert(self, loc, column, value):

C:\Python\Python32\lib\site-packages\pandas\core\generic.pyc in _set_item(self, key, value)
    580
    581     def _set_item(self, key, value):
--> 582         self._data.set(key, value)
    583         self._clear_item_cache()
    584

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in set(self, item, value)
   1467         except KeyError:
   1468             # insert at end
-> 1469             self.insert(len(self.items), item, value)
   1470
   1471         self._known_consolidated = False

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in insert(self, loc, item, value)
   1480
   1481             # new block
-> 1482             self._add_new_block(item, value, loc=loc)
   1483
   1484         except:

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in _add_new_block(self, item, value, loc)
   1519             loc = self.items.get_loc(item)
   1520         new_block = make_block(value, self.items[loc:loc + 1].copy(),
-> 1521                                self.items, fastpath=True)
   1522         self.blocks.append(new_block)
   1523

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in make_block(values, items, ref_items, klass, fastpath)
    859             klass = ObjectBlock
    860
--> 861     return klass(values, items, ref_items, ndim=values.ndim, fastpath=fastpath)
    862
    863 # TODO: flexible with index=None and/or items=None

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in __init__(self, values, items, ref_items, ndim, fastpath)
    693             values = np.array(values, dtype=object)
    694
--> 695         super(ObjectBlock, self).__init__(values, items, ref_items, ndim=ndim, fastpath=fastpath)
    696
    697     @property

C:\Python\Python32\lib\site-packages\pandas\core\internals.pyc in __init__(self, values, items, ref_items, ndim, fastpath)
     38         if len(items) != len(values):
     39             raise ValueError('Wrong number of items passed %d, indices imply %d'
---> 40                              % (len(items), len(values)))
     41
     42         self._ref_locs = None

ValueError: Wrong number of items passed 1, indices imply 40