On Wed, Jun 8, 2016 at 9:38 PM, Andrew Svetlov <andrew.svetlov@gmail.com> wrote:

My first thought was to use `async with` but I found that just `with` also works.

Sorry, but I don't follow how async context manager may be better.

`async with` communicates directly with the coroutine runner. Plain `with` requires that the coroutine runner be discoverable in some other way (via a global). Either way, for `asyncio.timeout` to be portable to other coroutine runners, we must define a protocol that coroutine runners may implement.

If we use `async with`, that protocol is something like "if the coroutine yields an instance of asyncio.TimeoutSetter, the runner should set its timeout accordingly".

If we use plain `with`, we would need to move asyncio.Task._current_task somewhere public (an attribute of EventLoop? a global threading.local?) and the protocol would be that each coroutine runner must register itself there while it is on the top of the stack and implement a `set_timeout()` method.

I believe the `async with` solution will perform better (an isinstance() check on every yielded object, plus one extra yielded object when the timeout feature is used, versus at least two operations on a central registry every time the coroutine runner enters or leaves the stack), and it also degrades more gracefully (the TimeoutSetter object could be a subclass of Future that resolves instantly so runners that don't understand it can simply ignore it. A runner that doesn't participate in the current_task registry may leave the wrong task at the top of the stack).

-Ben

On Wed, Jun 8, 2016 at 10:49 AM Yury Selivanov <yselivanov@gmail.com> wrote:

> On Jun 8, 2016, at 1:15 PM, Andrew Svetlov <andrew.svetlov@gmail.com> wrote:
>
> asyncio.timeout behaves like asyncio.wait_for BTW.
> It cancels inner tasks and sends asyncio.TimeoutError to outer.

Right. And I think it’s a totally normal behaviour for asyncio.

The only thing I’d fix in asyncio.timeout() (aside from resolving
Ben’s questions) is to make it use ‘async with’ statement.

Yury

--
Thanks,
Andrew Svetlov

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