Indefinitely cycle through some sequence in Python, e.g. for object pools.

loop.py

# loop foreva-eva
def loop(sequence):
    while True:
        for elem in sequence:
            yield elem


# example usage, imagine you have a set of things you want
# to cycle through, removing bad ones as they appear
class ObjectPoolEmptyError(Exception):
    pass


class ObjectPool(object):
    def __init__(self, object_sequence):
        self.__valid = set(object_sequence)
        self.__pool = loop(self.__valid)

    def checkout_object_from_pool(self):
        # obviously, you need locking of some kind...
        if len(self.__valid) > 0:
            return self.__pool.next()
        else:
            raise ObjectPoolEmptyError()
        # release locking

    def report_bad_object(self, bad_obj):
        # obviously requires locking...
        try:
            self.__valid.remove(bad_obj)
        except KeyError:
            pass

        # update pool
        self.__pool = loop(self.__valid)
        # release locking

if __name__ == '__main__':
    ducks = ['Daffy', 'Donald', 'Rufus']
    pool = ObjectPool(ducks)
    for i in range(len(ducks) * 4):
        duck = pool.checkout_object_from_pool()
        print i, duck
        if i % 3 == 2:
            print 'removing %s' % duck
            pool.report_bad_object(duck)

# 0 Donald
# 1 Daffy
# 2 Rufus
# removing Rufus
# 3 Donald
# 4 Daffy
# 5 Donald
# removing Donald
# 6 Daffy
# 7 Daffy
# 8 Daffy
# removing Daffy
# Traceback (most recent call last):
#   File "loop.py", line 42, in <module>
#     duck = pool.checkout_object_from_pool()
#   File "loop.py", line 24, in checkout_object_from_pool
#     raise ObjectPoolEmptyError()
# __main__.ObjectPoolEmptyError