set - Generate all subsets of size k (containing k elements) in Python

Question

I have a set of values and would like to create list of all subsets containing 2 elements.

For example, a source set ([1,2,3]) has the following 2-element subsets:

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

Is there a way to do this in python?

Answer 1

Seems like you want itertools.combinations:

>>> list(itertools.combinations((1, 2, 3), 2))
[(1, 2), (1, 3), (2, 3)]

If you want sets you'll have to convert them explicitly. If you don't mind an iterable instead of a list, and you're using Python 3, you can use map:

>>> s = set((1, 2, 3))
>>> map(set, itertools.combinations(s, 2))
<map object at 0x10cdc26d8>

To view all the results at once, you can pass the output of map to list. (In Python 2, the output of map is automatically a list.)

>>> list(map(set, itertools.combinations(s, 2)))
[{1, 2}, {1, 3}, {2, 3}]

However, if you know you'll need a list, a list comprehension is marginally better (h/t Jacob Bowyer):

>>> [set(i) for i in itertools.combinations(s, 2)]
[{1, 2}, {1, 3}, {2, 3}]