Twisted Python - If deferred goes out of scope, will it ever be fired?

Question

If I create a Deferred and add some callbacks to it while in the reactor event loop, will it ever be called if I let the local reference go out of scope? For example, if I have a protocol with a connectionMade() like so:

def connectionMade(self):
    # This made up function returns a deferred that will connect to 
    # some remote server using some made up protocol and return some data.
    d = connectToRemoteServer(reactor, url)
    d.addCallback(self._handleRemoteConnection)
    # At this point, d is going to go out of scope, so will the deferred
    # it points to ever get fired?

def _handleRemoteConnection(self, data):
    # Do something with the data

I have seen this pattern in different code that uses Twisted, and I am having trouble understanding why the deferred that is returned from connectToRemoteServer() is not garbage collected when d goes out of scope. I would think it would never fire, or maybe fail randomly due to a race condition. Can anyone explain to me why this works? I have read http://twistedmatrix.com/documents/current/core/howto/defer.html a couple times, but I still am not sure why this would work?

Thanks,

Carl

Answer 1

The hypothetical connectToRemoteServer API is going to hold a reference to d internally, usually with a reference from the global reactor to an object which will fire d (by calling d.callback) when the operation represented by d is complete.

So the reference goes from the stack, to the stack frame for reactor.run (because the reactor is running), to d.