[Twisted-Python] A Python metaclass for Twisted allowing __init__ to return a Deferred

Tristan Seligmann mithrandi at mithrandi.net
Sun Nov 2 19:41:56 EST 2008

* Terry Jones <terry at jon.es> [2008-11-03 01:16:50 +0100]:

> > I usually solve this kind of problem like this:
> > 
> >     d = aFuncReturningADeferred()
> >     d.addCallback(MyClass)
> >     d.addCallback(cb)
> See? I knew there'd be a 3-line solution somewhere :-)
> That's very cute, I'd never have thought of that...  Python is so nice.
> My (weak?) response is that your solution isn't quite so general. What if I
> want __init__ to generate the deferred based on its args, or on the result
> of other functions called based on its args? Yes, I can do all that work
> before using your lovely d.addCallback(MyClass), but then the logic of what
> the class does is starting to leak out into the code that should be calling
> the class. And you can't encapsulate THAT code into another class, because
> its __init__ would in turn have to return a deferred.

You can, however, encapsulate it into a classmethod or free function
that you call to get an instance of MyClass. This is a pattern often
seen when there are various ways to build an instance of some class;
you'll have a very basic __init__, and then a bunch of factory
functions or classmethods that do more complicated work before passing
the results in to the class's constructor.

> BTW, I sometimes have the feeling that Deferreds are like how M$
> characterizes the GPL. Viral. Once you call anything creating a Deferred,
> you're hooked - whatcha gonna do with that Deferred boy? Take one sip of
> the Deferred Kool Aid and the next thing you know you're on a drip.

Just wait until you start trying to combine iterators and deferreds ;)
mithrandi, i Ainil en-Balandor, a faer Ambar
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