A class instance as a coroutine

[davidbrochart]

I'd like to have a class instance look like a coroutine:

class Foo:
    def __await__(self):
        return self.run().__await__()

    async def run(self):
        print("run")

But AnyIO doesn't recognize it as a coroutine:

import anyio

async def main():
    async with anyio.create_task_group() as tg:
        tg.start_soon(Foo)

anyio.run(main)

# TypeError: Expected __main__.Foo() to return a coroutine, but the return value (<__main__.Foo object at 0x72557fdc28a0>) is not a coroutine object

Is there a better way to write my class so that AnyIO accepts it as a task function?

[uSpike]

Not sure if your example has a typo or not, but you're passing the Foo class and not an instance. You have to pass an instance of Foo.

[davidbrochart]

Thanks @smurfix, strangely your Foo implementation still doesn't work on AnyIO/asyncio, but it works on Trio:

import anyio

async def main():
    async with anyio.create_task_group() as tg:
        tg.start_soon(Foo)

anyio.run(main, backend="trio")

[smurfix]

Huh. Works for me, yes also with asyncio backend. Python 3.11.2. What's your version (and your error backtrace)?

[davidbrochart]

I tried with Python 3.11.2, same error:

Traceback (most recent call last):
  File "/home/david/git/anycorn/foo.py", line 24, in main
    tg.start_soon(Foo)
  File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 777, in start_soon
    self._spawn(func, args, name)
  File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 754, in _spawn
    raise TypeError(
TypeError: Expected __main__.Foo() to return a coroutine, but the return value (<__main__.Foo object at 0x73cc37b92f50>) is not a coroutine object

During handling of the above exception, another exception occurred:

  + Exception Group Traceback (most recent call last):
  |   File "/home/david/git/anycorn/foo.py", line 27, in <module>
  |     anyio.run(main, backend="asyncio")
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_core/_eventloop.py", line 74, in run
  |     return async_backend.run(func, args, {}, backend_options)
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 2034, in run
  |     return runner.run(wrapper())
  |            ^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/asyncio/runners.py", line 118, in run
  |     return self._loop.run_until_complete(task)
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/asyncio/base_events.py", line 653, in run_until_complete
  |     return future.result()
  |            ^^^^^^^^^^^^^^^
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 2022, in wrapper
  |     return await func(*args)
  |            ^^^^^^^^^^^^^^^^^
  |   File "/home/david/git/anycorn/foo.py", line 22, in main
  |     async with anyio.create_task_group() as tg:
  |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 680, in __aexit__
  |     raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/home/david/git/anycorn/foo.py", line 24, in main
    |     tg.start_soon(Foo)
    |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 777, in start_soon
    |     self._spawn(func, args, name)
    |   File "/home/david/micromamba/envs/anycorn/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 754, in _spawn
    |     raise TypeError(
    | TypeError: Expected __main__.Foo() to return a coroutine, but the return value (<__main__.Foo object at 0x73cc37b92f50>) is not a coroutine object

It boils down to inspect.iscoroutine(Foo()) being False, instead of True.

[smurfix]

Sorry about this; I had patched out that test and my editor refused to restore the original without telling me about it. Sigh.

asyncio is not using typing.Coroutine for this test for the simple reason that its code is somewhat older than typing.

AFAIK unless the asyncio people accept a patch that changes this you're out of luck here.

[davidbrochart]

though frankly I'd recommend being somewhat more explicit and using tg.start_soon(Foo().run) instead.

Thanks, I guess I'll be more explicit then 😄

[davidbrochart]

asyncio is not using typing.Coroutine for this test for the simple reason that its code is somewhat older than typing.

Actually, as I said it fails because AnyIO is using inspect.iscoroutine(Foo()), so it's not related to asyncio?
And the code for inspect.iscoroutine is:

def iscoroutine(object):
    """Return true if the object is a coroutine."""
    return isinstance(object, types.CoroutineType)

But a class cannot inherit from types.CoroutineType, which is defined here as:

async def _c(): pass
_c = _c()
CoroutineType = type(_c)
_c.close()  # Prevent ResourceWarning

[smurfix]

Yeah, you're right, I seem to have gotten confused. s/asyncio/inspect.

[davidbrochart]

Actually asyncio doesn't use inspect.iscoroutine(Foo()), only AnyIO does.
So this works with asyncio:

import asyncio
import typing

class Foo(typing.Coroutine):
    def __await__(self):
        return self.run().__await__()

    def send(self, value=None):
        return self.__await__().send(value)

    def throw(self, *_unused):
        raise RuntimeError("Shouldn't happen")

    async def run(self):
        print("run")

async def main():
    foo = Foo()
    task = asyncio.create_task(foo)
    await task

asyncio.run(main())

And this works with Trio:

import typing

import trio

class Foo(typing.Coroutine):
    def __await__(self):
        return self.run().__await__()

    def send(self, value=None):
        return self.__await__().send(value)

    def throw(self, *_unused):
        raise RuntimeError("Shouldn't happen")

    async def run(self):
        print("run")

async def main():
    async with trio.open_nursery() as nursery:
        nursery.start_soon(Foo)

trio.run(main)

But this doesn't work with AnyIO:

import typing

import anyio

class Foo(typing.Coroutine):
    def __await__(self):
        return self.run().__await__()

    def send(self, value=None):
        return self.__await__().send(value)

    def throw(self, *_unused):
        raise RuntimeError("Shouldn't happen")

    async def run(self):
        print("run")

async def main():
    async with anyio.create_task_group() as tg:
        tg.start_soon(Foo)

anyio.run(main)