Pandas reset inner level of MultiIndex

Question

I have a DF in the following format:

                   col1    col2
ID          Date
 1    1993-12-31      4       6
      1994-12-31      8       5
      1995-12-31      4       7
      1996-12-31      3       3
 2    2000-12-31      7       8
      2001-12-31      5       9
      2002-12-31      8       4

And I want to reset the 'Date' index giving the following:

             col1    col2
ID    Date
 1       0      4       6
         1      8       5
         2      4       7
         3      3       3
 2       0      7       8
         1      5       9
         2      8       4

I thought simply df.reset_index(level='Date', inplace=True, drop=True) would do it, but it does not.

Answer 1

Using pd.MultiIndex.from_arrays and groupby + cumcount.

df.index = pd.MultiIndex.from_arrays(
    [df.index.get_level_values(0), df.groupby(level=0).cumcount()],
    names=['ID', 'Date'])

df
         col1  col2
ID Date            
1  0        4     6
   1        8     5
   2        4     7
   3        3     3
2  0        7     8
   1        5     9
   2        8     4

This won't generalise to N levels, but there should be a df.index.set_levels equivalent I'm forgetting...