One of my favourite Twisted patterns is the combination of the powerful twisted.internet.defer.Deferred with collections.defaultdict.

Start with the right imports:

from collections import defaultdict
from twisted.internet.defer import Deferred

Now, a defaultdict allows you to say:

a = defaultdict(lambda: 0)
a[10] += 1
print a[10] # => 1

That is, you pass it a function (lambda: 0 is just the function that returns zero every time you call it) which returns the default value for the values of the mapping, and then you can access the keys of the mapping and get the default value for a key which doesn’t yet exist in the mapping, instead of a KeyError.

Combining this with deferreds allows an extraordinarily powerful mechanism to attach event handlers to arbitrary events which may not have happened yet. Here’s a contrived example (recall, in Python a class constructor is just a callable which returns an instance of that class, which is why we don’t wrap Deferred in a lambda):

on_mount = defaultdict(Deferred)

# ... I want to do something if filesystem abc ever gets mounted...

def hooray(arg):
	print "My favourite filesystem got mounted - %s" % str(arg)

on_mount['abc'].addCallback(hooray)

Now, we can do this without ever knowing that ‘abc’ might get mounted (the filesystem name might come from the network), or pre-populating the dictionary at all.

Later on in the code, we might actually mount ‘abc’:

def mount_handler(filesystem):
	on_mount[filesystem].callback(True)
	on_mount[filesystem] = Deferred() # reset the deferred

The beauty of it is that any number of event handlers (zero or more) can be attached to on_mount, with no additional house-keeping. All the callbacks attached to the deferred will get fired when the filesystem gets mounted.

The only thing to notice is that we have to reset the deferred if this event might be triggered again, as deferreds are one-shot. If it’s a one-off event, you can omit this, and then you get the additional nice behaviour that if you addCallback to a deferred after it’s been fired, you get an immediate callback with None as the argument, to indicate that the deferred has already run (from which you can deduce that the filesystem is already mounted).