Actually, I have a dataset about a "meeting". For example, A,B,C have a meeting, then the list would be [A,B,C]. Like this, each list would contain a list of members who participated in the meeting. Therefore:

line1= (A,B,C)

line2= (A,C,D,E)

line3 = (D,F,G)

...

I just would like to count the number how many times each pair of members meet each other. For example, member A meets C two times from line1 and line2 and member B meets C one time from line1. So, I would like to make a chart like this..

    A  B  C  D E F G...A  .  1  2  1 ...  B  1  . 1  0 C

...

I thought it would be easy at the first but I am pretty confused. Please help me and thank you so much in advance.

Rather than manually summing frequencies, use collections.counter along with itertools:

from collections import Counter
from itertools import chain, combinationsmeets = Counter(chain.from_iterable(combinations(line, 2) for line in lines))

Where lines is an iterable of iterables of names.