Getting the subsets of a set in Python

Question

Suppose we need to write a function that gives the list of all the subsets of a set. The function and the doctest is given below. And we need to complete the whole definition of the function

``````def subsets(s):
"""Return a list of the subsets of s.

>>> subsets({True, False})
[{False, True}, {False}, {True}, set()]
>>> counts = {x for x in range(10)} # A set comprehension
>>> subs = subsets(counts)
>>> len(subs)
1024
>>> counts in subs
True
>>> len(counts)
10
"""
assert type(s) == set, str(s) + ' is not a set.'
if not s:
return [set()]
element = s.pop()
rest = subsets(s)
``````

It has to not use any built-in function

My approach is to add "element" into rest and return them all, but I am not really familiar how to use set, list in Python.

1
6
11/3/2011 12:32:05 AM

Look at the powerset() recipe in the itertools docs.

``````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))

def subsets(s):
return map(set, powerset(s))
``````
15
11/3/2011 12:01:36 AM