How to re-index as multi-index pandas dataframe from index value that repeats

Question

I have an index in a pandas dataframe which repeats the index value. I want to re-index as multi-index where repeated indexes are grouped.

The indexing looks like such:

so I would like all the 112335586 index values would be grouped under the same in index.

I have looked at this question Create pandas dataframe by repeating one row with new multiindex but here the value can be index can be pre-defined but this is not possible as my dataframe is far too large to hard code this.

I also looked at at the multi-index documentation but this also pre-defines the value for the index.

Answer 1

I believe you need:

s = pd.Series([1,2,3,4], index=[10,10,20,20])
s.index.name = 'EVENT_ID'
print (s)
EVENT_ID
10    1
10    2
20    3
20    4
dtype: int64

s1 = s.index.to_series()
s2 = s1.groupby(s1).cumcount()
s.index = [s.index, s2]
print (s)
EVENT_ID   
10        0    1
          1    2
20        0    3
          1    4
dtype: int64

Answer 2

Try this:

df.reset_index(inplace=True)
df['sub_idx'] = df.groupby('EVENT_ID').cumcount()
df.set_index(['EVENT_ID','sub_idx'], inplace=True)