Implementing an asynchronous iterator

问题

Per PEP-492 I am trying to implement an asynchronous iterator, such that I can do e.g.

async for foo in bar:
    ...

Here is a trivial example, similar to the one in the docs, with a very basic test of instantiation and async iteration:

import pytest

class TestImplementation:
    def __aiter__(self):
        return self
    async def __anext__(self):
        raise StopAsyncIteration


@pytest.mark.asyncio  # note use of pytest-asyncio marker
async def test_async_for():
    async for _ in TestImplementation():
        pass

However, when I execute my test suite, I see:

=================================== FAILURES ===================================
________________________________ test_async_for ________________________________

    @pytest.mark.asyncio
    async def test_async_for():
>       async for _ in TestImplementation():
E       TypeError: 'async for' received an invalid object from __aiter__: TestImplementation

...: TypeError
===================== 1 failed, ... passed in 2.89 seconds ======================

Why does my TestImplementation appear to be invalid? As far as I can tell it meets the protocol:

1. An object must implement an __aiter__ method ... returning an asynchronous iterator object.
2. An asynchronous iterator object must implement an __anext__ method ... returning an awaitable.
3. To stop iteration __anext__ must raise a StopAsyncIteration exception.

This is failing with the latest released versions of Python (3.5.1), py.test (2.9.2) and pytest-asyncio (0.4.1).

回答1:

If you read a little further down the documentation it mentions that (emphasis mine):

PEP 492 was accepted in CPython 3.5.0 with __aiter__ defined as a method, that was expected to return an awaitable resolving to an asynchronous iterator.

In 3.5.2 (as PEP 492 was accepted on a provisional basis) the __aiter__ protocol was updated to return asynchronous iterators directly.

Therefore for versions prior to 3.5.2 (released 2016/6/27) the documentation is slightly out of step with how to write a working asynchronous iterator. The fixed version for 3.5.0 and 3.5.1 looks like:

class TestImplementation:
    async def __aiter__(self):
  # ^ note
        return self
    async def __anext__(self):
        raise StopAsyncIteration

This was introduced on closing bug #27243 and is a little clearer in the data model documentation, which also suggests a way of writing backwards compatible code.

回答2:

Asynchronous iterators have been implemented in Python 3.6 - see PEP-525

Then you don't need your TestImplementation at all in order to use async for. You can just use yield (example taken from PEP-525):

async def ticker(delay, to):
    """Yield numbers from 0 to `to` every `delay` seconds."""
    for i in range(to):
        yield i
        await asyncio.sleep(delay)

You can then use async for as you would expect:

async for i in ticker(1, 10):                                                                     
    print(f'Tick #{i}')

来源：https://stackoverflow.com/questions/38031066/implementing-an-asynchronous-iterator