RE: [Twisted-Python] Question on deferreds

Dec. 5, 2005

      Ken wrote:
...
Hi Frank,
Best that I can tell, it's not your various classes that need 
to be converted to deferreds, just your program that calls 
class1, or even class1 itself.
Thanks for the reply.
...
The bottom line is that _something_ higher up the calling 
stack needs to be aware that this is an asynchronous process 
- and that a deferred gets generated along the way. I believe 
it would be helpful if the originating function is coded for this.
I understand what you are getting at, but I have spent a few hours trying to
squeeze this concept into my program, and I am not succeeding.

There are two main reasons that I can see.

Firstly, I oversimplified my pseudo code.

You suggested changing this

    class1():
        def check1(data):
            perform test
            if test failed:
                return False
            if not class2.check2(data):
                return False
            return True

to this

     class1():
         def check1(data):
             perform test
             if test failed:
                 return defer.succeed(False)
             return class2.check2(data)

I understand that, but I should have used this as an example -

    class1():
        def check1(data):
            perform test
            if test failed:
                return False
            if not class2.check2(data):
                return False
            do some extra stuff here  # this is what I left out
            return True

This 'extra stuff' can occur at any of the levels in the stack. I cannot see
where I could put this using your version.

Secondly, under some circumstances I loop over a number of objects and
validate them one after the other -

    class1():
        def check1(data):
            for obj in objList:
                if not check2(obj):
                    return False
            perform test
            if test failed:
                return False
            if not check2(data):
                return False
            do some extra stuff here
            return True

As you can see, it is getting even more complicated to do this using
deferreds.

The irony is that I actually want the client program to block until a
response is received, so maybe I should not be using Twisted on the client
at all. I could write a normal socket client, and use a ServerFactory on the
server.

I am very new to Twisted, so it is likely that my problem is that I have not
learned to think in a Twisted manner yet. Any advice to put me straight will
be much appreciated.

Thanks

Frank

RE: [Twisted-Python] Question on deferreds

Frank Millman