[Python-Dev] PEP 492: async/await in Python; version 4

[Yury Selivanov]

Stefan,

I don't like the idea of combining __next__ and __anext__.
In this case explicit is better than implicit.  __next__
returning coroutines is a perfectly normal thing for a
normal 'for' loop (it wouldn't to anything with them),
whereas 'async for' will interpret that differently, and
will try to await those coroutines.

Yury

On 2015-05-01 1:10 PM, Stefan Behnel wrote:
> Guido van Rossum schrieb am 01.05.2015 um 17:28:
>> On Fri, May 1, 2015 at 5:39 AM, Stefan Behnel wrote:
>>
>>> Yury Selivanov schrieb am 30.04.2015 um 03:30:
>>>> Asynchronous Iterators and "async for"
>>>> --------------------------------------
>>>>
>>>> An *asynchronous iterable* is able to call asynchronous code in its
>>>> *iter* implementation, and *asynchronous iterator* can call
>>>> asynchronous code in its *next* method.  To support asynchronous
>>>> iteration:
>>>>
>>>> 1. An object must implement an  ``__aiter__`` method returning an
>>>>     *awaitable* resulting in an *asynchronous iterator object*.
>>>>
>>>> 2. An *asynchronous iterator object* must implement an ``__anext__``
>>>>     method returning an *awaitable*.
>>>>
>>>> 3. To stop iteration ``__anext__`` must raise a ``StopAsyncIteration``
>>>>     exception.
>>> What this section does not explain, AFAICT, nor the section on design
>>> considerations, is why the iterator protocol needs to be duplicated
>>> entirely. Can't we just assume (or even validate) that any 'regular'
>>> iterator returned from "__aiter__()" (as opposed to "__iter__()") properly
>>> obeys to the new protocol? Why additionally duplicate "__next__()" and
>>> "StopIteration"?
>>>
>>> ISTM that all this really depends on is that "__next__()" returns an
>>> awaitable. Renaming the method doesn't help with that guarantee.
>>
>> This is an astute observation. I think its flaw (if any) is the situation
>> where we want a single object to be both a regular iterator and an async
>> iterator (say when migrating code to the new world). The __next__ method
>> might want to return a result while __anext__ has to return an awaitable.
>> The solution to that would be to have __aiter__ return an instance of a
>> different class than __iter__, but that's not always convenient.
> My personal gut feeling is that this case would best be handled by a
> generic wrapper class. Both are well defined protocols, so I don't expect
> people to change all of their classes and instead return a wrapped object
> either from __iter__() or __aiter__(), depending on which they want to
> optimise for, or which will eventually turn out to be easier to wrap.
>
> But that's trying to predict the [Ff]uture, obviously. It just feels like
> unnecessary complexity for now. And we already have a type slot for
> __next__ ("tp_iternext"), but not for __anext__.
>
> Stefan
>
>
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