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

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　　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)