Snippet with metaclass enabling a class to have an async __init__ method in Python.

awaitable_class.py

import inspect
from functools import wraps

def async_init_wrapper(func):
    def wrapper_stage1(instance):
        async def wrapper_stage2(*args, **kw):
            value = await func(instance, *args, **kw)
            if value is not None:
                raise TypeError("__async_init__() should return None")
            return instance
        return wrapper_stage2
    wrapper_stage1.__name__ = func.__name__
    return wrapper_stage1


class AwaitableClass(type):
    def __new__(mcls, name, bases, ns, **kw):
        if "__init__" in ns and inspect.iscoroutinefunction(ns["__init__"]):
            ns["__async_init__"] = async_init_wrapper(ns.pop("__init__"))
        return super().__new__(mcls, name, bases, ns, **kw)

    def __call__(cls, *args, **kw):
        instance = super().__call__(*args, **kw)
        if not isinstance(instance, cls) or not hasattr(cls, "__async_init__"):
            return instance
        return instance.__async_init__()(*args, **kw)


def test_awaitable_class():
    import asyncio

    class Server(metaclass=AwaitableClass):

        async def __init__(self):
            self.connection = await self.connect()

        async def connect(self):
            await asyncio.sleep(1)
            return 'server initialized connection'

    async def concurrent_task():
        await asyncio.sleep(0.5)
        print("doing stuff while server is initialized")

    async def init_server():
        print("starting server initialization")
        server_instance = await Server()
        print("server ready")
        return server_instance

    async def main():
        results = await asyncio.gather(
            init_server(),
            concurrent_task()
        )
        return results[0]
    loop = asyncio.get_event_loop()
    server_instance = loop.run_until_complete(main())
    print(server_instance.connection)


if __name__ == "__main__":

    test_awaitable_class()