Generating all possible combinations of a list, "itertools.combinations" misses some results

Question

Given a list of items in Python, how can I get all the possible combinations of the items?

There are several similar questions on this site, that suggest using itertools.combinations, but that returns only a subset of what I need:

stuff = [1, 2, 3]
for L in range(0, len(stuff)+1):
    for subset in itertools.combinations(stuff, L):
        print(subset)

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

As you see, it returns only items in a strict order, not returning (2, 1), (3, 2), (3, 1), (2, 1, 3), (3, 1, 2), (2, 3, 1), and (3, 2, 1). Is there some workaround for that? I can't seem to come up with anything.

Answer 1

Use itertools.permutations:

>>> import itertools >>> stuff = [1, 2, 3] >>> for L in range(0, len(stuff)+1):         for subset in itertools.permutations(stuff, L):                 print(subset) ...          () (1,) (2,) (3,) (1, 2) (1, 3) (2, 1) (2, 3) (3, 1) .... 

Help on itertools.permutations:

permutations(iterable[, r]) --> permutations object  Return successive r-length permutations of elements in the iterable.  permutations(range(3), 2) --> (0,1), (0,2), (1,0), (1,2), (2,0), (2,1) 

Answer 2

You can generate all the combinations of a list in python using this simple code

import itertools  a = [1,2,3,4] for i in xrange(1,len(a)+1):    print list(itertools.combinations(a,i)) 

Result:

[(1,), (2,), (3,), (4,)] [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)] [(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)] [(1, 2, 3, 4)] 

Answer 3

Are you looking for itertools.permutations instead?

From help(itertools.permutations),

Help on class permutations in module itertools:

class permutations(__builtin__.object)
 |  permutations(iterable[, r]) --> permutations object
 |  
 |  Return successive r-length permutations of elements in the iterable.
 |  
 |  permutations(range(3), 2) --> (0,1), (0,2), (1,0), (1,2), (2,0), (2,1)

Sample Code :

>>> from itertools import permutations
>>> stuff = [1, 2, 3]
>>> for i in range(0, len(stuff)+1):
        for subset in permutations(stuff, i):
               print(subset)


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

From Wikipedia, the difference between permutations and combinations :

Permutation :

Informally, a permutation of a set of objects is an arrangement of those objects into a particular order. For example, there are six permutations of the set {1,2,3}, namely (1,2,3), (1,3,2), (2,1,3), (2,3,1), (3,1,2), and (3,2,1).

Combination :

In mathematics a combination is a way of selecting several things out of a larger group, where (unlike permutations) order does not matter.