Operation on every pair of element in a list

Check out product() in the itertools module. It does exactly what you describe.

import itertools

my_list = [1,2,3,4]
for pair in itertools.product(my_list, repeat=2):
foo(*pair)


This is equivalent to:

my_list = [1,2,3,4]
for x in my_list:
for y in my_list:
foo(x, y)


Edit: There are two very similar functions as well, permutations() and combinations(). To illustrate how they differ:

product() generates every possible pairing of elements, including all duplicates:

1,1  1,2  1,3  1,4
2,1  2,2  2,3  2,4
3,1  3,2  3,3  3,4
4,1  4,2  4,3  4,4


permutations() generates all unique orderings of each unique pair of elements, eliminating the x,x duplicates:

 .   1,2  1,3  1,4
2,1   .   2,3  2,4
3,1  3,2   .   3,4
4,1  4,2  4,3   .


Finally, combinations() only generates each unique pair of elements, in lexicographic order:

 .   1,2  1,3  1,4
.    .   2,3  2,4
.    .    .   3,4
.    .    .    .


All three of these functions were introduced in Python 2.6.