[Twisted-Python] Multiple deferreds that depend on each other

[Landreville]

Oh I like the look of the inlineCallbacks. Is there a way that I can still
use inspect.getargspec to retrieve the arg names of my original method, as
opposed to the generator returned by inlineCallbacks.

I noticed it copies the __doc__ over, but is there an attribute where I can
still inspect my original method?

On Tue, Nov 17, 2009 at 11:08 AM, Reza Lotun <rlotun at gmail.com> wrote:

> Hi Landreville,
>
> > If I want to do a number of actions in a row that each return a deferred
> but
> > depend on eachother, do I need to nest those callbacks?
> > Ie if i want to:
> > 1. Call database function using adbabi
> > 2. Call xmlrpc method using result from 1
> > 3. Call database using result from 2
> > Would I just put 2 inside the callback for 1 and then put 3 inside the
> > callback for 2?
>
> By multiple deferreds that depend on each other, you probably mean a
> linear firing of actions based on callbacks of previous actions. The
> only way to do that is to trigger actions in callbacks of the
> deferreds in question, as you allude to.
>
> Yeah, so roughly something like this would work:
>
> def db_call(arg):
>   dfr = make_db_call_in_thread(arg)
>   return dfr
>
> def xmlrpc_call(arg):
>   dfr = make_call(arg)
>   return dfr
>
> dfr = db_call(some_initial_arg)
> dfr.addCallback(xmlrpc)
> dfr.addCallback(db_call)
>
> > How would I do that in one function? All the examples I have seen have a
> > function calling one method returning a deferred and then placing the
> logic
> > (doing something with the result) in the callback, but do not deal with
> > multiple deferreds that depend on each other.
>
> To do it in one function, you could use something like:
>
> def complex_call():
>    def db_call(arg):
>       dfr = make_db_call_in_thread(arg)
>       return dfr
>
>    def xmlrpc_call(arg):
>       dfr = make_call(arg)
>       return dfr
>
>    dfr = db_call(some_initial_arg)
>    dfr.addCallback(xmlrpc_call)
>    dfr.addCallback(db_call)
>    dfr.addErrback(my_errback_handler)
>    return dfr
>
> Or you can make each handler a method on a class. Another way to do it
> is to use inlineCallbacks:
>
> from twisted.internet import defer
> @defer.inlineCallbacks
> def example():
>    res = yield db_call(some_initial_arg)
>    rpc_res = yield xmlrpc_call(res)
>    db_res = yield db_call(rpc_res)
>
> In the above example, it's all still asynchronous (if not parallel),
> but it looks like it's synchronous, using the magic of decorators and
> Python 2.5+'s ability to retrieve results from yield statements.
>
> Hope that makes sense.
>
> Cheers,
> Reza
>
> --
> Reza Lotun
> mobile: +44 (0)7521 310 763
> email:  rlotun at gmail.com
> work:   reza at tweetdeck.com
> twitter: @rlotun
>
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