Python combinations of list elements with condition

Question

I find source code of itertools.combinations() function in python. It looks like this.

def combinations(iterable, r):
pool = tuple(iterable)
n = len(pool)
if r > n:
    return
indices = list(range(r))
print(indices)
yield tuple(pool[i] for i in indices)
while True:
    for i in reversed(range(r)):
        if indices[i] != i + n - r:
            break
    else:
        return
    indices[i] += 1
    for j in range(i+1, r):
        indices[j] = indices[j-1] + 1
    print(indices)
    yield tuple(pool[i] for i in indices)

I have tuples like this:

pairs = [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]

I need to generate foursomes of all possible combinations, but with condition, that there will be always only two same numbers in list. So in this case this 3 list I want to generate:

((0, 1), (0, 2), (1, 3), (2, 3))
((0, 1), (0, 3), (1, 2), (2, 3))
((0, 2), (0, 3), (1, 2), (1, 3))

What I realy need is to update code of generation combinations, because in my real app I need to generate 23-nties from 80 tuples. Generation and filtering after it would take a lot of time, thats why I need to catch problem in part of generation.

Answer 1

You can use itertools.combinations and then filter the result using collections.Counter :

from collections import Counter
import itertools as it

pairs = [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]
result = filter(
    lambda x: max(Counter(it.chain(*x)).values()) < 3,
    it.combinations(pairs, 4)
)
print(list(result))

Output:

[((0, 1), (0, 2), (1, 3), (2, 3)),
 ((0, 1), (0, 3), (1, 2), (2, 3)),
 ((0, 2), (0, 3), (1, 2), (1, 3))]