Allow Return Statements with Values in Asynchronous Generators

Feature or enhancement

Proposal:

async def gen():
    yield 1
    yield 2
    return 3


async def runner():
    g = gen()
    while True:
        try:
            a = await g.__anext__()
            print("yielded", a)
        except StopAsyncIteration as e:
            print("returned", e.value)
            break


asyncio.run(runner())


# yielded 1
# yielded 2
# returned 3

The proposal adds optional return values to async generators.

This feature should be implemented to achieve syntactic and functional parity with synchronous generators.

Today:

def gen():
  yield 1
  yield 2
  return 3

async def agen():
  yield 1
  yield 2
  return 3 # Syntax error

Proposal:

def gen():
  yield 1
  yield 2
  return 3

async def agen():
  yield 1
  yield 2
  return 3

It can be implemented trivially in CPython as a nonbreaking change.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/allow-return-statements-with-values-in-asynchronous-generators/66886

Linked PRs

• gh-125401

Start by reading the linked Discourse discussion and linked PR gh-125401 to understand the proposed language and runtime changes. Use the async-generator example in the issue as the behavioral reference; done means an async generator can return a value and that value is observable through StopAsyncIteration without breaking existing behavior.