import random

class grab_bag(object):
    def __init__(self):
        self.contents = set()

    def clear(self):
        self.contents.clear()

    def add(self, item):
        if item not in self.contents:
            self.contents.add(item)

    def add_all(self, collection):
        for x in collection:
            self.add(x)

    def subset(self, count):
        if len(self.contents) < count:
            return None
        subset = random.sample(self.contents, count)
        return subset

    def size(self):
        return len(self.contents)

#x = grab_bag()
#x.add_all([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
#print x.subset(3)