Pandas: all possible combinations of rows

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I have a DataFrame looking like..

ID   c1   c2   cX
r1   2    3    ..   
r2   8    9    ..
rY   ..

I want to generate a new DataFrame with all possible (two-part) combinations of rows whilst concatenating the columns of the two combined rows (so that the new DF would have twice as much columns). The result should look like:

ID       c1_r1   c1_r2   c2_r1   c2_r2   cX_rA
r1_r2    2        8        3       9       ..
r1_r3    ..      ..       ..       ..
rA_rB    ..

The ID name isn't important (it could even be a MultiIndex) nor is the order of the columns of importance.

How to approach this?

pandas combinations concatenation python
1 Answers

Consider df

    c1  c2
ID        
r1   2   3
r2   8   9
r3   0   7

I'd do it like this

from itertools import combinations

a, b = map(list, zip(*combinations(df.index, 2)))

print(a, b, sep='\n')

['r1', 'r1', 'r2']
['r2', 'r3', 'r3']

Then use pd.concat

d = pd.concat(
    [df.loc[a].reset_index(), df.loc[b].reset_index()],
    keys=['a', 'b'], axis=1
)

d

    a         b      
   ID c1 c2  ID c1 c2
0  r1  2  3  r2  8  9
1  r1  2  3  r3  0  7
2  r2  8  9  r3  0  7

Finally, tie up loose ends

d.set_index([('a', 'ID'), ('b', 'ID')]).rename_axis(['a', 'b'])

       a     b   
      c1 c2 c1 c2
a  b             
r1 r2  2  3  8  9
   r3  2  3  0  7
r2 r3  8  9  0  7
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