I'm trying to figure out if our documentation on the new awaitable concept
in Python 3.6+ is correct. It seems to imply that if an object's __await__
method returns an iterator, the object is awaitable. However, just
returning an iterator doesn't seem to work with await in a coroutine or
with the asyncio selector loop's run_until_complete method.
If the awaitable is not a coroutine or future, it looks like we wrap it in
a coroutine using sub-generator delegation, and therefore have to have an
iterator that fits a very specific shape for the coroutine step process
that isn't documented anywhere I could find. Am I missing something?
If the definition of an awaitable is more than just an __await__ iterator,
we may need to expand the documentation as well as the abstract base class.
Here's what I tried in making a synchronous awaitable that resolves to the
int 42:
class MyAwaitable(Awaitable):
def __await__(self):
return iter((42,))
# RuntimeError: Task got bad yield: 42
class MyAwaitable(Awaitable):
def __await__(self):
yield 42
# RuntimeError: Task got bad yield: 42
class MyAwaitable(Awaitable):
def __await__(self):
return (i for i in (42,))
# RuntimeError: Task got bad yield: 42
class MyAwaitable(Awaitable):
def __await__(self):
return self
def __next__(self):
return 42
# RuntimeError: Task got bad yield: 42'''
class MyAwaitable(Awaitable):
def __await__(self):
return iter(asyncio.coroutine(lambda: 42)())
# TypeError: __await__() returned a coroutine
class MyAwaitable(Awaitable):
def __await__(self):
yield from asyncio.coroutine(lambda: 42)()
# None
class MyAwaitable(Awaitable):
def __await__(self):
return (yield from asyncio.coroutine(lambda: 42)())
# 42
async def await_things():
print(await MyAwaitable())
asyncio.get_event_loop().run_until_complete(await_things())
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