[Twisted-Python] Deferred documentation rewrite

Kevin Horn kevin.horn at gmail.com
Thu Jul 30 17:45:54 EDT 2009

On Thu, Jul 30, 2009 at 4:27 PM, Terry Jones <terry at jon.es> wrote:

> > Right.  Everything you could do synchronously, you can do asynchronously
> > (and a bit more too).  It just /looks/ really weird.
> At some point I realized that it helps to realize that code that looks
> synchronous really isn't at all. Probably all the programming that almost
> all of us will ever do is not synchronous. You type
>  x = y + 3
> and you get into the habit of thinking that x just got assigned the value
> of y + 3. But there's a whole set of levels of operations, all invisible to
> you, that are frantically rearranging atoms behind the scenes to pull off
> this tiny miracle. Instead of trying to wrap your head around the so-called
> "weird" asynchronous operation of Twisted, it's easier to let fall the
> comforting illusion that you're in the synchronous world to begin with -
> whatever that's supposed to mean (it's more like a fairy tale IMO). You're
> not.
> It's more accurate to think that that line of code arranges for x to
> *eventually* hold the value of y + 3. Then it's a small step to thinking
> that
>  x = y + 3
>  z = f(x)
> is just another slightly bigger part of the same illusion. Those lines of
> code aren't executed one after another at all. You're just brought up with
> the fairy tale that they are - or at least you study computer science and
> spend so much time with high-level languages that you start to take it for
> granted that they are done one after the other. But the second line is
> *eventually* executed after the first, assuming nothing went wrong (e.g., y
> could be None).
> At that point - everything is asynchronous - you have the right mindset to
> think perfectly clearly about Twisted code. And there's no jump to make at
> all. The only things you have to get your head around are 1) the fact that
> Twisted code doesn't give you the comforting linear illusion of so-called
> synchronous code as you type it in your editor (inlineCallbacks helps a lot
> with improving that), and 2) that Twisted's deferreds are the way of
> maintaining the flow of results (both correct and incorrect) through time
> as your program executes. I.e., you've lost the step-at-a-time line-by-line
> results computed on line N are available on line N+1 flow of code you can
> so easily read in your editor, and in exchange you have deferreds.
> I could go on, obviously :-) But I just wanted to point out that the shift
> from thinking "synchronously" to "asynchronously" is much easier to make if
> you realize the former is a convenient illusion and in fact you've always
> been doing things asynchronously.
> Terry

Thanks for this, Terry.  I'd never thought of it that way, and it's quite a
good point.

Kevin Horn
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