[Python-ideas] The async API of the future: Twisted and Deferreds

[Itamar Turner-Trauring]

On Sat, Oct 13, 2012 at 11:46 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>
> > In a Deferred callback, on the other hand, you know the only things that
> are
> > going to run are functions you call. In so far as it's possible, what
> > happens is under control of one function only. Less pretty, but no
> potential
> > race conditions:
> >
> >     def add(result):
> >         counter.value = counter.value + result
> >     getResult().addCallback(add)
>
> This is not the same code you wrote above in the generator version.
> The callback equivalent of the code you wrote is this:
>
>     bound_value = counter.value
>     def add(result):
>         counter.value = bound_value + result
>     getResult().addCallback(add)
>

True, so, let's look at this version. First, notice that it's more
convoluted than the version I wrote above; i.e. you have to go out of your
way to write race conditiony code. Second, and much more important, when
reading it it's obvious that you're getting and setting counter.value at
different times! Whereas in the generator version you have to think about
it. The generator version has you naturally writing code where things you
thought are happening at the same time are actually happening very far
apart; the Deferred code makes it clear which pieces of code happen
separately, and so you're much more likely to notice these sort of bugs.

The generator version isn't magic, people still need to know what
> they're doing to properly benefit from the cooperative multithreading.
>

I agree. And that's exactly the dimension in which Deferreds are superior
to cooperative multithreading; people don't have to think about race
conditions as much, which is hard enough  in general. At least when you're
using Deferreds, you can tell by reading the code which chunks of code can
happen at different times, and the natural idioms of Python don't
*encourage* race conditions as they do with yield syntax.

-Itamar
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