On Wed, Jan 28, 2004 at 07:22:40PM +0100, Fabrice Coudert wrote:
Tommi Virtanen a ?crit :
Fabrice Coudert wrote:
I 'd like to add a pre-processing and post-processing callback in a deferred callback list. I did somthing like this :
my_defer = getADeferredForSomething() new_defer = defer.Deferred() new_defer.addCallback(preprocessing) new_defer.chainDeferred(my_defer) new_defer.addCallback(postprocessing) my_defer = new_defer
You are using Deferreds incorrectly. For example, nothing here makes new_defer ever trigger.
It is a sample code to show inconcistency of chainDeferred, not a real case. Add my_defer.callback(True) after the sample code if you'd like and trace it. You should see that postprocessing() is called with None argument, but not with the result of previous callback as it should be with a normal behaviour of deferred callbacks (read the tutorial)
Without a use case, there's no chance of the behavior changing. False consistency is the hobgoblin of, etc.
You haven't stated clearly what you want to do, but I don't see why you aren't just doing this:
d = getADeferredForSomething() d.addCallback(preprocessing) d.addCallback(theRealMcCoy) d.addCallback(postprocessing)
Same as before, it is a *sample code*. In real case theRealMcCoy (and some more methodes) is added before I need to add a post/preprocessing to that defer.
If the code you included in your message isn't equivalent to the code you're using in your actual application, then it's a waste of time to discuss it. If it is equivalent, I don't see why you can't use the code glyph suggested.
So, I restate the probleme with chainDeferred : chainDeferred(d) is equivalent to addCallbacks(d.callback, d.errback), but this two methods are not really sort of callback methods for deferred coz they didn't return a result.
If it doesn't do what you want, don't use it. For that matter, even if it does, you can probably avoid using it. As glyph stated, it is a hold-over from times past. If you can share the real use case, maybe someone can help you reformulate it in terms of more recent APIs.
effcy.
Jp