getting every possible combination in a list

Question

suppose I had something like this:

L1=['cat', 'dog', 'fish', 'rabbit', 'horse', 'bird', 'frog', 'mouse'...]

for x in L1:
    input1= open('file_%s'%(x), 'r')
    file1= pickle.load(input1)
    for x in L1:
        input2= open('file_%s'%(x), 'r')
        file2= pickle.load(input2)

and I wanted to get every combination of files without repeating combinations that have already been done (once cat_dog is done do not do dog_cat again). Is there a way I could do this? My real list is in alphabetical order, if that makes any difference.

Answer 1

In reality what you're asking how to do is produce all combinations of two items taken in the list of names (as opposed to all the possible combination of them).

That means you can use the built-in itertools.combinations() generator function to easily (and efficiently) generate pairs of the names you want with no repeats:

L1 = ['cat', 'dog', 'fish', 'rabbit', 'horse', 'bird', 'frog', 'mouse']

for pair in combinations(L1, 2):
    print(pair)
    input1 = open('file_%s' % pair[0], 'r')
    input2 = open('file_%s' % pair[1], 'r')

Pairs processed:

('cat', 'dog')
('cat', 'fish')
('cat', 'rabbit')
('cat', 'horse')
('cat', 'bird')
('cat', 'frog')
('cat', 'mouse')
('dog', 'fish')
('dog', 'rabbit')
('dog', 'horse')
('dog', 'bird')
('dog', 'frog')
('dog', 'mouse')
('fish', 'rabbit')
('fish', 'horse')
('fish', 'bird')
('fish', 'frog')
('fish', 'mouse')
('rabbit', 'horse')
('rabbit', 'bird')
('rabbit', 'frog')
('rabbit', 'mouse')
('horse', 'bird')
('horse', 'frog')
('horse', 'mouse')
('bird', 'frog')
('bird', 'mouse')
('frog', 'mouse')

Answer 2

you can also do it as a generator:

L1=['cat', 'dog', 'fish', 'rabbit', 'horse', 'bird', 'frog', 'mouse']
tuples = [(x,y) for x in L1 for y in L1 if x != y]
for entry in tuples:
    if (entry[1], entry[0]) in tuples:
        tuples.remove((entry[1],entry[0]))
for pair in tuples:
    input1= open('file_%s'%(pair[0]), 'r')
    file1= pickle.load(input1)
    input2= open('file_%s'%(pair[1]), 'r')
    file2= pickle.load(input2)

After the first loop, the contents of tuples is:

('cat', 'dog')
('cat', 'fish')
('cat', 'rabbit')
('cat', 'horse')
('cat', 'bird')
('cat', 'frog')
('cat', 'mouse')
('dog', 'fish')
('dog', 'rabbit')
('dog', 'horse')
('dog', 'bird')
('dog', 'frog')
('dog', 'mouse')
('fish', 'rabbit')
('fish', 'horse')
('fish', 'bird')
('fish', 'frog')
('fish', 'mouse')
('rabbit', 'horse')
('rabbit', 'bird')
('rabbit', 'frog')
('rabbit', 'mouse')
('horse', 'bird')
('horse', 'frog')
('horse', 'mouse')
('bird', 'frog')
('bird', 'mouse')
('frog', 'mouse')