[Twisted-Python] If the errbacks of a canceled Deferred are called with error other than CancelledError, is this acceptable?

[Terry Jones]

Hi Tom

> Since most apis in twisted don't pass a canceller, the behavior is a2+a3.

Right. That was the intention (glyph's). It was expected that over time
more Twisted apis that create deferreds would add cancellers. As I
understand it, Kai Zhang is now adding cancellers to some apis returning
deferreds, starting with the POP3 client.

Based on JP's comment, I still think I'm missing something here.

The only suggestion I've been making is that, if we wanted to, we could
allow a value to be passed to the cancel call. That would allow errback
functions to receive more information when a Deferred is canceled. You'd
still get a CancelledError in any case, but the value would be in the
exception instance as args[0], as in

In [1]: from twisted.internet.defer import CancelledError
In [2]: e = CancelledError('hey')
In [3]: e.args[0]
Out[3]: 'hey'


Terry


On Fri, Jun 21, 2013 at 6:12 AM, Tom Prince <tom.prince at ualberta.net> wrote:

> Terry Jones <terry at jon.es> writes:
> > OK, maybe someone can explain the original "Well, it already raises
> > `CancelledError`. Every deferred that doesn't have explicitly handle
> > cancelling already does:" to me, because I didn't get it at all!
>
> This is what happens when a deferred is cancelled:
>
> a) If the deferred hasn't been errbacked:
>    1. Call the provied canceller (provided when the deferred is
> constructed)
>    2. If a canceller isn't provided, or if the canceller didn't
>       callback/errback, then errback with CancelledError.
>    3. If a canceller isn't provided, allow callback/errback to be called
>       without error once.
> b) If the deferred has been fired, and is waiting on another deferred,
>    cancel that deferred instead.
>
> Since most apis in twisted don't pass a canceller, the behavior is
> a2+a3.
>
>   Tom
>
>
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