[Python-Dev] Musings on concurrency and scoping ("replacing" Javascript)

[Bob Ippolito]

On Jul 6, 2006, at 5:04 PM, Ka-Ping Yee wrote:

> On Thu, 6 Jul 2006, Phillip J. Eby wrote:
>> As much as I'd love to have the nested scope feature, I think it's  
>> only
>> right to point out that the above can be rewritten as something  
>> like this
>> in Python 2.5:
>>
>>      def spam():
>>          local_A = do_work()
>>          result_1 = yield do_network_transaction()
>>          local_B = do_work(result_1)
>>          result_2 = yield do_network_transaction()
>>          do_work(local_A, local_B, result_1, result_2)
>>          ...
>>
>> All you need is an appropriate trampoline (possibly just a  
>> decorator) that
>> takes the objects yielded by the function, and uses them up to set up
>> callbacks that resume the generator with the returned result.
>
> Clever!  Could you help me understand what goes on in
> do_network_transaction() when you write it this way?  In the
> Firefox/JavaScript world, the network transaction is fired off
> in another thread, and when it's done it posts an event back
> to the JavaScript thread, which triggers the callback.
>
> And what happens if you want to supply more than one continuation?
> In my JavaScript code i'm setting up two continuations per step --
> one for success and one for failure, since with a network you never
> know what might happen.

When you have a failure the yield expression raises an exception  
instead of returning a result.

-bob