python permutation list

1def permutations(iterable, r=None):
2    # permutations('ABCD', 2) --> AB AC AD BA BC BD CA CB CD DA DB DC
3    # permutations(range(3)) --> 012 021 102 120 201 210
4    pool = tuple(iterable)
5    n = len(pool)
6    r = n if r is None else r
7    if r > n:
8        return
9    indices = list(range(n))
10    cycles = list(range(n, n-r, -1))
11    yield tuple(pool[i] for i in indices[:r])
12    while n:
13        for i in reversed(range(r)):
14            cycles[i] -= 1
15            if cycles[i] == 0:
16                indices[i:] = indices[i+1:] + indices[i:i+1]
17                cycles[i] = n - i
18            else:
19                j = cycles[i]
20                indices[i], indices[-j] = indices[-j], indices[i]
21                yield tuple(pool[i] for i in indices[:r])
22                break
23        else:
24            return
25print ( list(permutations([1, 2, 3], 2)) )
26# [(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]

1import itertools
2
3a = [1, 2, 3]
4n = 3
5
6perm_iterator = itertools.permutations(a, n)
7
8for item in perm_iterator:
9    print(item)