Operation on every pair of element in a list

Question

Hello everyone,

Using Python, I'd like to compare every possible pair in a list.

Suppose I have

my_list = [1,2,3,4]

I'd like to do an operation (let's call it foo) on every combination of 2 elements from the list.

The final result should be the same as

foo(1,1)
foo(1,2)
...
foo(4,3)
foo(4,4)

My first thought was to iterate twice through the list manually, but that doesn't seem very pythonic.

Answer 1

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.

Answer 2

If you're just calling a function, you can't really do much better than:

for i in my_list:
    for j in my_list:
        foo(i, j)

If you want to collect a list of the results of calling the function, you can do:

[foo(i, j) for i my_list for j in my_list]

which will return you a list of the result of applying foo(i, j) to each possible pair (i, j).