[Twisted-Python] Cancel/stop Deferred() at any time

[vitaly at synapticvision.com]

Thank you,

"cancel" callback - stop running now what you've been asked to exec.
Example: some SQL query that takes 5 min and I need to cancel it  
anywhere in the middle and it, of course, it doesn't matter what the  
result set.

As far as I see from base.py and client.py, some sort of timeout now  
could achieve the cancel operation ?


Quoting "Jean-Paul Calderone" <exarkun at divmod.com>:

> On Tue, 14 Jul 2009 11:10:10 -0400, vitaly at synapticvision.com wrote:
>> Hello,
>>
>> is there any way to cancel the Deferred callback in those examples?
>>
>> 1.
>> d = defer.Deferred()
>> d.addCallback(methodA).addErrback(methodErr)
>> d.callback(None)
>>
>> 2.
>> d = methodB().addErrback(methodErr)
>> wfd = defer.waitForDeferred(d)
>>
>>
>>
>> Appreciate any help.
>>
>
> Deferred doesn't have any cancellation features.  You can generally achieve
> some sort of cancellation by supporting this in the application code creating
> and consuming the Deferreds involved.
>
> The details of how to do this depend on what exactly you want to happen when
> you "cancel" the operation.
>
> You can find one example of this in the implementation of Twisted's internal
> thread-based name resolver:
>
> http://twistedmatrix.com/trac/browser/trunk/twisted/internet/base.py#L193
>
> You can find other examples in the implementation of any other Deferred-
> returning API which also supports timeouts, eg the web client:
>
> http://twistedmatrix.com/trac/browser/trunk/twisted/web/client.py#L198
>
> Hope this helps,
>
> Jean-Paul
>
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