> On Nov 16, 2016, at 3:35 PM, Nathaniel Smith <[email protected]> wrote:
>
> What's the use case for the async generator version? Could the yield be
> replaced by 'await loop.shutting_down()’?
Async generator version (inspired by contextlib.contextmanager decorator) is
needed in cases where you want loop.run_forever.
The PR originally proposed to add `asyncio.forever()` (which is the same idea
as `loop.shutting_down()`), but nobody particularly liked it.
A couple of thoughts/reasons:
1. Some pretty intrusive modifications are required to be made in the event
loop to make it work. That means all other event loops (including uvloop) will
have to be modified to support it. This is the most important reason.
2. `loop.shutting_down()` is a no go because it’s a method on the loop object.
We can discuss `asyncio.shutting_down`. The whole point of this discussion is
to get rid of the event loop.
3. `await forever()` and `await shutting_down()` have a naming issue - both
look weird:
async def main():
srv = await asyncio.start_server(…)
try:
await asyncio.shutting_down() # or await forever()
finally:
srv.close()
await srv.wait_closed()
In the above example, what does the second ‘await’ do? Will it be resolved
when the loop is stopped with ‘loop.stop()’? Or when a KeyboardInterrupt
occurs? What will happen if you await on it in parallel from 10 different
coroutines? It’s just difficult to define a clear semantics of this coroutine.
Using an asynchronous generator for this case is easier in terms of
implementation and in terms of specifying the execution semantics. And the
approach of using generators for such things isn’t new - we have
contextlib.contextmanager decorator which is quite popular.
Yury
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