Commutative combination of elements of two lists

Question

Just a style question: Is there a build-in method to get the combinations under the assertion of commutative property and excluding elements paired with itself?

Answer 1

This will work if you do not care that the ordering in the resulting tuples does not map to the input lists (you do not care whether (1,2) or (2,1)). Here you'll get the combination with the smaller element first:

a = [1,2,3]
b = [1,3,4,5]

set([(min(x,y), max(x,y)) for x in a for y in b if x != y])

gives

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

With strings

a = '1 2 3'.split()
b = '1 3 4 5'.split()

you get

set([('2', '3'),
     ('3', '5'),
     ('1', '4'),
     ('3', '4'),
     ('1', '5'),
     ('1', '2'),
     ('2', '5'),
     ('1', '3'),
     ('2', '4')])

The apparent difference in the ordering comes from the different hashes for the strings and the numbers.