<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ngroup(self, ascending=True):
 """ Number each group from 0 to the number of groups - 1. This is the enumerative complement of cumcount. Note that the numbers given to the groups match the order in which the groups would be seen when iterating over the groupby object, not the order they are first observed. .. versionadded:: 0.20.2 Parameters ascending : bool, default True If False, number in reverse, from number of group - 1 to 0. See Also -------- .cumcount : Number the rows in each group. Examples -------- A 0 a 1 a 2 a 3 b 4 b 5 a 0 0 1 0 2 0 3 1 4 1 5 0 dtype: int64 0 1 1 1 2 1 3 0 4 0 5 1 dtype: int64 0 0 1 0 2 1 3 3 4 2 5 0 dtype: int64 """


        with _group_selection_context(self):
            index = self._selected_obj.index
            result = Series(self.grouper.group_info[0], index)
            if not ascending:
                result = self.ngroups - 1 - result
            return result