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Does something like this do what you're looking for?

sage: s = set([1,3,"two"]) # make a set from a list of items
sage: t = list(subsets(s)); t  # this is the list of all subsets of s
[[], [1], [3], [1, 3], ['two'], [1, 'two'], [3, 'two'], [1, 3, 'two']]
sage: for x in subsets(s):  # subsets(s) is an iterator, so you can loop through it
....:     if len(x) == 2:   # and extract items of a specified length
....:         print x
....:         
[1, 3]
[1, 'two']
[3, 'two']