for i in DIFFERENCE:
       for j in DIFFERENCE:
           if(some condition and i!=j and i+'_'+j not in COMPARISON and j+'_'+i not in COMPARISON):
               COMPARISON.append(i+'_'+j);
               COMPARISON.append(j+'_'+i);
               ANS_COUNT=ANS_COUNT+1;

>>> import itertools
>>> list(itertools.combinations([1,2,3,4,5,6], 2))
[(1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (2, 3), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6), (4, 5), (4, 6), (5, 6)]

l = list()
for i in range (1,7):
    for j in range (2,7):
        if(i == j):
            j = j + 1
        elif ((i,j) in l or (j,i) in l):
            continue
        else:
            l.append((i,j))

print l

[(1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (2, 3), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6), (4, 5), (4, 6), (5, 6)]