[Python-Dev] async/await in Python; v2

[Guido van Rossum]

On Wed, Apr 22, 2015 at 1:10 PM, Andrew Svetlov <andrew.svetlov at gmail.com>
wrote:

> On Wed, Apr 22, 2015 at 10:44 PM, PJ Eby <pje at telecommunity.com> wrote:
> > On Tue, Apr 21, 2015 at 1:26 PM, Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
> >> It is an error to pass a regular context manager without ``__aenter__``
> >> and ``__aexit__`` methods to ``async with``.  It is a ``SyntaxError``
> >> to use ``async with`` outside of a coroutine.
> >
> > I find this a little weird.  Why not just have `with` and `for` inside
> > a coroutine dynamically check the iterator or context manager, and
> > either behave sync or async accordingly?  Why must there be a
> > *syntactic* difference?
>
> IIRC Guido always like to have different syntax for calling regular
> functions and coroutines.
> That's why we need explicit syntax for asynchronous context managers
> and iterators.
>

To clarify: the philosophy behind asyncio coroutines is that you should be
able to tell statically where a task may be suspended simply by looking for
`yield from`. This means that *no* implicit suspend points may exist, and
it rules out gevent, stackless and similar microthreading frameworks.

In the new PEP this would become `await`, plus specific points dictated by
`async for` and `async with` -- `async for` can suspend (block) at each
iteration step, and `async with` can suspend at the enter and exit points.
The use case for both is database drivers: `async for` may block for the
next record to become available from the query, and `async with` may block
in the implied `finally` clause in order to wait for a commit. (Both may
also suspend at the top, but that's less important.)

-- 
--Guido van Rossum (python.org/~guido)
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