On Fri, May 1, 2015 at 5:39 AM, Stefan Behnel <stefan...@behnel.de> 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. Thus, aware of the choice, I would still prefer a separate __anext__ method. -- --Guido van Rossum (python.org/~guido)
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