[Python-ideas] PEP 530: Asynchronous Comprehensions

[Sven R. Kunze]

On 06.09.2016 19:38, Yury Selivanov wrote:
>
>
> On 2016-09-06 9:40 AM, C Anthony Risinger wrote:
>> On Tue, Sep 6, 2016 at 10:20 AM, Sven R. Kunze <srkunze at mail.de 
>> <mailto:srkunze at mail.de>> wrote:
>>
>>         So, what's the "async" good for then?
>>
>>
>> Maybe I'm off base here, but my understanding is the `async for` 
>> version would allow for suspension during the actual iteration, ie. 
>> using the __a*__ protocol methods, and not just by awaiting on the 
>> produced item itself.
>>
>> IOW, `[await ... async for ...]` will suspend at least twice, once 
>> during iteration using the __a*__ protocols and then again awaiting 
>> on the produced item, whereas `[await ... for ...]` will 
>> synchronously produce items and then suspend on them. So to use the 
>> async + await version, your (async) iterator must return awaitables 
>> to satisfy the `async for` part with then produce another awaitable 
>> we explicitly `await` on.
>>
>> Can someone confirm this understanding? And also that all 4 
>> combinations are possible, each with different meaning:
>>
>> # Normal comprehension, 100% synchronous and blocking
>> [... for ...]
>>
>> # Blocking/sync iterator producing awaitables which suspend before 
>> producing a "normal" value
>> [await ... for ...]
>>
>> # Non-blocking/async iterator that suspends before producing "normal" 
>> values
>> [... async for ...]
>>
>> # Non-blocking/async iterator that suspends before producing 
>> awaitables which suspend again before producing a "normal" value
>> [await ... async for ...]
>>
>> Is this correct?
>
> All correct.  I'll update the PEP to better clarify the semantics.

Still don't understand what the "async" is good for. Seems redundant to me.


Sven