GH-97696: Initial work on eager tasks

[gvanrossum]

This defines an event loop method eager_task_factory() that can be made the task factory using loop.set_task_factory(loop.eager_task_factory). It will then return a Future if the coroutine completed (or failed) without ever suspending itself.

As a TEMPORARY demonstration (to be revisited), for now we call this factory from TaskGroup.create_task(), if the loop implementation defines it.

• Issue: Add eager task creation API to asyncio #97696

[1st1]

Hm, this doesn't have to be a loop-level method? I imagined this to be a top-level function in the asyncio package.

One would install it like this:

async def main():
    loop = asyncio.get_running_loop()
    loop.set_task_factory(asyncio.eager_task_factory)
    
    # ... the rest of the main() function ...

asyncio.run(main())

[gvanrossum]

I didn't try very hard to make that work yet... The problem is that the new factory has to call the old factory, but (at least in the Python version) the old factory defaults to None and then create_task() just constructs an instance of the Task class. But when there's a different loop I don't know how to do the equivalent thing -- can I just assume that get_task_factory() shouldn't return None if they have a different task class? (Is uvloop's factory ever None?)

[1st1]

The problem is that the new factory has to call the old factory

Why? I don't think this is a requirement. I remember we discussed it with the instagram people and the idea was to have two functions in asyncio:

# The custom eager factory API that instagram wants to use because
# they have a bunch of instrumentation on top of asyncio.Task:
def asyncio.create_eager_task_factory(custom_task_constructor):

    def factory(coro):
        try:
            result = coro.send(None)
        except StopIteration as si:
             f = Future()
             f.set_result(si.value)
             return f
        except Exception as ex:
             f = Future()
             f.set_exception(ex)
             return f
        else:
             return custom_task_constructor(coro, coro_result=result)

    return factory

# The default eager factory API that most users will use:
asyncio.eager_task_factory = create_eager_task_factory(asyncio.Task)

The implementation of loop.set_task_factory and loop.create_task methods would stay the same. We'd have to add the coro_result= keyword-only argument to asyncio.Task (both Python and C versions), but that I think is acceptable.

I just assume that get_task_factory() shouldn't return None if they have a different task class? (Is uvloop's factory ever None?)

I think None is just a marker for the "default factory is used" scenario. I don't think we were ever wanted to chain factories.

---

Overall I really want this to be implemented not on the event loop, the loop API surface is already very large.

[gvanrossum]

Oh, that's a nice solution. If I get back to this before anyone else I'll use that.