On Wed, Apr 22, 2015 at 1:10 PM, Andrew Svetlov <andrew.svet...@gmail.com> wrote:
> On Wed, Apr 22, 2015 at 10:44 PM, PJ Eby <p...@telecommunity.com> wrote: > > On Tue, Apr 21, 2015 at 1:26 PM, Yury Selivanov <yselivanov...@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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