Is there a way to call an `async` python method from C++?

Question

We have a codebase in python which uses asyncio, and co-routines (async methods and awaits), what I'd like to do is to call one of these method from a C++ class which has been pulled into python (using pybind11)

Let's say there is this code:

class Foo:
  async def bar(a, b, c):
    # some stuff
    return c * a

Assuming that the code is being invoked from python and there is an io loop handling this, at some point, the code drops into C++ land where this bar method needs to be invoked - how does one await the result of this in C++?

Answer 1

It is possible to implement a Python coroutine in C++, but takes some work. You need to do what the interpreter (in static languages the compiler) normally does for you and transform your async function into a state machine. Consider a very simple coroutine:

async def coro():
    x = foo()
    y = await bar()
    baz(x, y)
    return 42

Invoking coro() doesn't run any of its code, but it produces an awaitable object which can be started and then resumed multiple times. (But you don't normally see these operations because they are transparently performed by the event loop.) The awaitable can respond in two different ways: by 1) suspending, or by 2) indicating that it is done.

Inside a coroutine await implements suspension. If a coroutine were implemented with a generator, y = await bar() would desugar to:

# pseudo-code for y = await bar()

_bar_iter = bar().__await__()
while True:
    try:
        _suspend_val = next(_bar_iter)
    except StopIteration as _stop:
        y = _stop.value
        break
    yield _suspend_val

In other words, await suspends (yields) as long as the awaited object does. The awaited object signals that it's done by raising StopIteration, and by smuggling the return value inside its value attribute. If yield-in-a-loop sounds like yield from, you're exactly right, and that is why await is often described in terms of yield from. However, in C++ we don't have yield (yet), so we have to integrate the above into the state machine.

To implement async def from scratch, we need to have a type that satisfies the following constraints:

• doesn't do much when constructed - typically it will just store the arguments it received
• has an __await__ method that returns an iterable, which can just be self;
• has an __iter__ which returns an iterator, which can again be self;
• has a __next__ method whose invocation implements one step of the state machine, with return meaning suspension and raising StopIteration meaning finishing.

The above coroutine's state machine in __next__ will consist of three states:

1. the initial one, when it invokes the foo() sync function
2. the next state when it keeps awaiting the bar() coroutine for as long as it suspends (propagating the suspends) to the caller. Once bar() returns a value, we can immediately proceed to calling baz() and returning the value via the StopIteration exception.
3. the final state which simply raises an exception informing the caller that the coroutine is spent.

So the async def coro() definition shown above can be thought of as syntactic sugar for the following:

class coro:
    def __init__(self):
        self._state = 0

    def __iter__(self):
        return self

    def __await__(self):
        return self

    def __next__(self):
        if self._state == 0:
            self._x = foo()
            self._bar_iter = bar().__await__()
            self._state = 1

        if self._state == 1:
            try:
                suspend_val = next(self._bar_iter)
                # propagate the suspended value to the caller
                # don't change _state, we will return here for
                # as long as bar() keeps suspending
                return suspend_val
            except StopIteration as stop:
                # we got our value
                y = stop.value
            # since we got the value, immediately proceed to
            # invoking `baz`
            baz(self._x, y)
            self._state = 2
            # tell the caller that we're done and inform
            # it of the return value
            raise StopIteration(42)

        # the final state only serves to disable accidental
        # resumption of a finished coroutine
        raise RuntimeError("cannot reuse already awaited coroutine")

We can test that our "coroutine" works using real asyncio:

>>> class coro:
... (definition from above)
...
>>> def foo():
...     print('foo')
...     return 20
... 
>>> async def bar():
...     print('bar')
...     return 10
... 
>>> def baz(x, y):
...     print(x, y)
... 
>>> asyncio.run(coro())
foo
bar
20 10
42

The remaining part is to write the coro class in Python/C or in pybind11.

Answer 2

This isn't pybind11, but you can call an async function directly from C. You simply add a callback to the future using add_done_callback. I assume pybind11 allows you to call python functions so the steps would be the same:

https://github.com/MarkReedZ/mrhttp/blob/master/src/mrhttp/internals/protocol.c

result = protocol_callPageHandler(self, r->func, request))

Now the result of an async function is a future. Just like in python you need to call create_task using the resulting future:

PyObject *task;
if(!(task = PyObject_CallFunctionObjArgs(self->create_task, result, NULL))) return NULL;

And then you need to add a callback using add_done_callback:

add_done_callback = PyObject_GetAttrString(task, "add_done_callback")
PyObject_CallFunctionObjArgs(add_done_callback, self->task_done, NULL)

self->task_done is a C function registered in python which will be called when the task is done.