Hrvoje Nikšić <hnik...@gmail.com> added the comment:
The issue is because the current documentation *doesn't* say that
"`asyncio.sleep()` always pauses the current task and switches execution to
another one", it just says that it "blocks for _delay_ seconds".
With that description a perfectly valid implementation could be further
optimized with:
async def sleep(delay):
if delay <= 0:
return
...
In which case `await sleep(0)` would *not* cause a task switch. And this is not
an unreasonable thing to expect because there are many other
potentially-switching situations in asyncio that sometimes don't cause a
switch, such as await `queue.get()` from a non-empty queue or await `await
stream.readline()` from a socket stream that has a line to provide.
The user who wants to implement a "yield control to event loop" has to look at
the source to find out how delay==0 is handled, and then they have to wonder if
it's an implementation detail. https://github.com/python/asyncio/issues/284
states that the behavior is explicit and here to stay, but that promise has
never made it into the actual documentation.
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<https://bugs.python.org/issue34476>
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