pool.py

class Pool(object):

    def __init__(self, factory, size, finalizer=None):
        self._factory = factory
        self._finalizer = finalizer or lambda x: None
        self._pool = list()
        self._pool_sema = threading.Semaphore()
        for _ in range(size):
            self.put(self._factory())

    def get(self):
        self._pool_sema.acquire()
        return self._pool.pop()

    def put(self, obj):
        self._pool.append(obj)
        self._pool_sema.release()

    def replace(self, obj):
        self._finalizer(obj)
        self.put(self._factory())