counting in pandas dataframe, for each a_i, a_j of A, count number of times that a_i and a_j are listed with a same b

Question

I have a big dataframe (1M lines) with two columns A,B.

For each couple a_i, a_j of A, I want to know the number of b in B such that there is both rows a_i,b and a_j,b

For example :

A	B
a1	b1
a2	b1
a3	b2
a1	b3
a3	b3
a4	b3
a2	b4
a1	b5
a3	b5
a3	b6
a4	b6

here I have, among others, the couple a1,a3 that share b3 and b5

The result would be the following matrix (that, by definition, is symetric) :

	a1	a2	a3	a4
a1	xx	1	2	1
a2	1	xx	0	0
a3	2	0	xx	2
a4	1	0	2	xx

I think that the following would work:

df = pd.DataFrame({'A' : ['a1','a2','a3','a1','a3','a4','a2','a1','a3','a3','a4'],
 'B':['b1','b1','b2','b3','b3','b3','b4','b5','b5','b6','b6']})

df_dum = df.set_index('A')['B'].str.get_dummies().reset_index()
df_dum = df_dum.groupby('A').sum()
np_cnt = df_dum.to_numpy()
np_mul = np.matmul(np_cnt,np_cnt.T)

but it takes way too much time and memory and does not run with my 1M rows. In addition, the diagonal is coumputed whereas I don't need it and I think that passing through the dummy is not so much a good idea especially because the resulting binary is really sparse.

But I don't have any more ideas...

What would you propose ?

EDIT:

for a little bit more of context, let's say that A are students and B are courses. In the end I want to know for any two students, how much courses they have together. And that for every couple of students that at least share a course. If that makes more sense :)

Answer 1

Try with itertools.permutations:

import itertools

sets = df.groupby('B')['A'].apply(lambda x : list(itertools.permutations(x, 2))).explode().tolist()
sets = pd.DataFrame(sets)

index = df["A"].unique()
output = pd.crosstab(sets[0],sets[1],rownames=[None],colnames=[None]).reindex(index).reindex(index, axis=1)

>>> output
    a1  a2  a3  a4
a1   0   1   2   1
a2   1   0   0   0
a3   2   0   0   2
a4   1   0   2   0

If you want to mask the cells where index and columns are the same with "xx":

output = output.mask(output.index.values[:,None] == output.columns.values[None,:]).fillna("xx")

>>> output
     a1   a2   a3   a4
a1   xx  1.0  2.0  1.0
a2  1.0   xx  0.0  0.0
a3  2.0  0.0   xx  2.0
a4  1.0  0.0  2.0   xx