generate all set partitions

partitions.py

def partitions(set_):
	if not set_:
		yield []
		return
	for i in xrange(2**len(set_)/2):
		parts = [set(), set()]
		for item in set_:
			parts[i&1].add(item)
			i >>= 1
		for b in partitions(parts[1]):
			yield [parts[0]]+b