[Twisted-Python] deferred proposed update

[Jp Calderone]

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
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