Python - passing modified callback to dispatcher - python-2.7

Question

Scrapy application, but the question is really about the Python language - experts can probably answer this immediately without knowing the framework at all.
I've got a class called CrawlWorker that knows how to talk to so-called "spiders" - schedule their crawls, and manage their lifecycle.
There's a TwistedRabbitClient that has-one CrawlWorker. The client only knows how to talk to the queue and hand off messages to the worker - it gets completed work back from the worker asynchronously by using the worker method connect_to_scrape below to connect to a signal emitted by a running spider:
def connect_to_scrape(self, callback):
self._connect_to_signal(callback, signals.item_scraped)
def _connect_to_signal(self, callback, signal):
if signal is signals.item_scraped:
def _callback(item, response, sender, signal, spider):
scrape_config = response.meta['scrape_config']
delivery_tag = scrape_config.delivery_tag
callback(item.to_dict(), delivery_tag)
else:
_callback = callback
dispatcher.connect(_callback, signal=signal)
So the worker provides a layer of "work deserialization" for the Rabbit client, who doesn't know about spiders, responses, senders, signals, items (anything about the nature of the work itself) - only dicts that'll be published as JSON with their delivery tags.
So the callback below isn't registering properly (no errors either):
def publish(self, item, delivery_tag):
self.log('item_scraped={0} {1}'.format(item, delivery_tag))
publish_message = json.dumps(item)
self._channel.basic_publish(exchange=self.publish_exchange,
routing_key=self.publish_key,
body=publish_message)
self._channel.basic_ack(delivery_tag=delivery_tag)
But if I remove the if branch in _connect_to_signal and connect the callback directly (and modify publish to soak up all the unnecessary arguments), it works.
Anyone have any ideas why?

Answer 1

So, I figured out why this wasn't working, by re-stating it in a more general context:
import functools
from scrapy.signalmanager import SignalManager
SIGNAL = object()
class Sender(object):
def __init__(self):
self.signals = SignalManager(self)
def wrap_receive(self, receive):
#functools.wraps(receive)
def wrapped_receive(message, data):
message = message.replace('World', 'Victor')
value = data['key']
receive(message, value)
return wrapped_receive
def bind(self, receive):
_receive = self.wrap_receive(receive)
self.signals.connect(_receive, signal=SIGNAL,
sender=self, weak=False)
def send(self):
message = 'Hello, World!'
data = {'key': 'value'}
self.signals.send_catch_log(SIGNAL, message=message, data=data)
class Receiver(object):
def __init__(self, sender):
self.sender = sender
self.sender.bind(self.receive)
def receive(self, message, value):
"""Receive data from a Sender."""
print 'Receiver received: {0} {1}.'.format(message, value)
if __name__ == '__main__':
sender = Sender()
receiver = Receiver(sender)
sender.send()
This works if and only if weak=False.
The basic problem is that when connecting to the signal, weak=False needs to be specified. Hopefully someone smarter than me can expound on why that's needed.