Powersets in Python using itertools

Solution 1:

itertools functions return iterators, objects that produce results lazily, on demand.

You could either loop over the object with a for loop, or turn the result into a list by calling list() on it:

from itertools import chain, combinations

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

for result in powerset([1, 2, 3]):
    print(result)

results = list(powerset([1, 2, 3]))
print(results)

You can also store the object in a variable and use the next() function to get results from the iterator one by one.

Solution 2:

Here's a solution using a generator:

from itertools import combinations

def all_combos(s):
    n = len(s)
    for r in range(1, n+1):
        for combo in combinations(s, r):
            yield combo

Solution 3:

Allen Downey's answer is what I would use, except I would replace the range with range(n+1) since the empty set is considered an element of the power set.

This code can easily be rewritten as a one liner:

(combo for r in range(len(s) + 1) for combo in combinations(s, r))

Your choice whether to surround it with () for an iterator or [] for a list.