asynchronous_constructor.py

import asyncio


class AsyncConstructorMeta(type):
    """Metaclass to support asynchronous constructors in Python.
    
    Basically we're exploiting the fact that __new__ can return anything in Python.
    So we're taking the old __init__ code, removing it from the class, and instead,
    we create a custom __new__ method that returns a coroutine wrapping the original
    constructor.

    This is horrible code. Please don't use this to launch rockets in outer space.
    """
    def __new__(metacls, name, bases, dct):
        new_dct = dict(dct)
        old_init = new_dct.pop("__init__")
        new_cls = super().__new__(metacls, name, bases, new_dct)

        def new(cls, *args, **kwargs):
            async def init():
                obj = object.__new__(new_cls)
                await old_init(obj, *args, **kwargs)
                return obj
            return init()

        new_cls.__new__ = new
        return new_cls


class Example(metaclass=AsyncConstructorMeta):
    async def __init__(self, x):
        await asyncio.sleep(1.0)
        self.x = x


async def run():
    a = await Example(x=42)
    print(a.x)


asyncio.run(run())