python __call__ 内置函数的使用以及在OpenStack中的用法

对象通过提供__call__(slef, [,*args [,**kwargs]])方法可以模拟函数的行为，如果一个对象x提供了该方法，就可以像函数一样使用它，也就是说x(arg1, arg2...) 等同于调用x.__call__(self, arg1, arg2) 。 

class DistanceForm(object):

    def __init__(self, origin):

        self.origin = origin

        print "origin :"+str(origin)

    def __call__(self, x):

        print "x :"+str(x)

p = DistanceForm(100)

p(2000)

输出

>>> 

origin :100

x :2000

举例：

ReplyWaiter初始化时会调用declare_direct_consumer函数生成consumer，传给改函数的第二个参数self，等同于传的self.__call__函数，最终这个参数赋值给consumer对象的callback变量。

class ReplyWaiter(object):
    def __init__(self, reply_q, conn, allowed_remote_exmods):
        self.conn = conn
        self.allowed_remote_exmods = allowed_remote_exmods
        self.msg_id_cache = rpc_amqp._MsgIdCache()
        self.waiters = ReplyWaiters()

        self.conn.declare_direct_consumer(reply_q, self)

        self._thread_exit_event = threading.Event()
        self._thread = threading.Thread(target=self.poll)
        self._thread.daemon = True
        self._thread.start()


    def __call__(self, message):
        message.acknowledge()
        incoming_msg_id = message.pop('_msg_id', None)
        self.waiters.put(incoming_msg_id, message)

    def declare_direct_consumer(self, topic, callback):
        """Create a 'direct' queue.
        In nova's use, this is generally a msg_id queue used for
        responses for call/multicall
        """
        self.declare_consumer(DirectConsumer, topic, callback)

class ConsumerBase(object):
    """Consumer base class."""

    def __init__(self, channel, callback, tag, **kwargs):
        """Declare a queue on an amqp channel.

        'channel' is the amqp channel to use
        'callback' is the callback to call when messages are received
        'tag' is a unique ID for the consumer on the channel

        queue name, exchange name, and other kombu options are
        passed in here as a dictionary.
        """
        self.callback = callback
        self.tag = six.text_type(tag)
        self.kwargs = kwargs
        self.queue = None
        self.reconnect(channel)