Pandas dataframe conditional inner join with itself

Question

I am searching for a way to inner join a column of a dataframe with itself, based on a condition. I have a large dataframe consisting of two colums, 'Group' and 'Person'. Now I would like to create a second dataframe, which has an entry for every person tuple, that has been in the same group. First dataframe:

    Group | Person
    a1    | p1
    a1    | p2
    a1    | p3
    a1    | p4
    a2    | p1

Output:

    Person1 | Person2 | Weight
    p1      | p2      | 1
    p1      | p3      | 1
    p1      | p4      | 1
    p2      | p3      | 1
    p2      | p4      | 1
    p3      | p4      | 1

The weight is increased, if a tuple of persons are part of multiple groups. So far, I was able to create a naive implementation, based on a sub dataframe and two for loops. Is there a more elegant and more importantly, a faster/builtin way to do so ?

My implentation so far:

    group = principals.iloc[i,0]

    sub = principals.loc[principals['Group'] == group]
    
    for j in range(len(sub)-1):
        for k in range (j+1,len(sub)):
            #check if tuple exists -> update or create new entry

I was thinking, whether there is a functionality similar to SQL inner join, based on the condition of the group being the same and then joining person against person. I could take care of the double p1|p1 entry in that case...

Many thanks in advance

Answer 1

combinations will give you the tuple pairs you are looking for. Once you get those you can explode the tuple combinations into rows. Then your weight is the group size of each pair - in this case 1 because they all exist only in one group.

import pandas as pd
import numpy as np
from itertools import combinations

df = pd.DataFrame({'Group': ['a1', 'a1', 'a1', 'a1', 'a2'],
 'Person': ['p1', 'p2', 'p3', 'p4', 'p1']})

df = (
    df.groupby('Group')['Person']
      .apply(lambda x: tuple(combinations(x,2)))
      .explode()
      .dropna()
      .reset_index()
)

df['Weight'] = df.groupby('Person').transform(np.size)
df[['Person1','Person2']] = df['Person'].apply(pd.Series)

df = df[['Person1','Person2','Weight']]

print(df)

Output

  Person1 Person2  Weight
0      p1      p2       1
1      p1      p3       1
2      p1      p4       1
3      p2      p3       1
4      p2      p4       1
5      p3      p4       1