Python 3.9: async_generator 不能用于 'await' 表达式

Question

I'm trying the following:

class Payload_Session_Generator:
    def __init__(self):
        pass

    async def __anext__(self):
        async for payload in generate_fb_payload():
            if type(payload) != str:
                yield payload
            else:
                StopAsyncIteration

    def __aiter__(self):
        return self

This is then passed as an instance to a different function and the __aiter__ method is called explicitly and _iter is an object of the above class:

chunk = await self._iter.__anext__()

This then yields the following error:

TypeError: object async_generator can't be used in 'await' expression

Answer 1

Change your code like

class PayloadSessionGenerator:
    def __init__(self):
        pass
    
    async def __aiter__(self):
        async for payload in generate_fb_payload():
            if type(payload) != str:
                yield payload
            else:
                StopAsyncIteration

try next call

x = Payload_Session_Generator()
chunk = await x.__anext__()

Answer 2

An asynchronous generator is similar to a regular generator: __anext__ must provide one element each time it is called. Since an (async) generator is an (async) iterator, it must return the object itself.

As such, __anext__ should use return instead of yield :

class Payload_Session_Generator:
    async def __anext__(self):
        async for payload in generate_fb_payload():
            if type(payload) != str:
                return payload            # return one element
            else:
                raise StopAsyncIteration  # raise at end of iteration

    def __aiter__(self):
        return self

Note that if a class uses just __anext__ and __aiter__ , it is simpler to write a proper async generator. An async generator is defined using async def and yield in the body:

async def payload_session_generator(self):
    async for payload in generate_fb_payload():
        if type(payload) != str:
            yield payload  # yield each element
        else:
            break          # exiting ends iteration

Answer 3

I had a different scenario:

async def get_body_file(
    request: Request,
) -> AsyncGenerator[str, None]:
    loop = asyncio.get_event_loop()
    bytes_written = 0

    try:
        with tempfile.NamedTemporaryFile(delete=False) as fp:
            async for chunk in request.stream():
                bytes_written += len(chunk)
                if bytes_written > MAX_FILE_SIZE:
                    raise HTTP_BAD_REQUEST(
                        "file-limit-exceeded",
                        f"The maximum file limit has been exceeded, maximum allowed: {MAX_FILE_SIZE} bytes.",
                    )
                await loop.run_in_executor(None, fp.write, chunk)

        if bytes_written == 0:
            raise HTTP_BAD_REQUEST("body-expected", "No data received")

        yield fp.name

    finally:
        try:
            await loop.run_in_executor(None, os.unlink, fp.name)
        except FileNotFoundError:
            pass

for me it was to await this way:

...///
file = await get_body_file(request).__anext__()
...///