async getitem with futures

example.py

import asyncio


class MyAsyncDict:

    async def async_getitem(self, fut, key):
        try:
            await asyncio.sleep(0.5)
            raise RuntimeError('oops')
        except Exception as e:
            fut.set_exception(e)
        else:
            fut.set_result(1)

    def __getitem__(self, key):
        loop = asyncio.get_event_loop()
        fut = loop.create_future()
        loop.create_task(self.async_getitem(fut, key))
        return fut


async def myfunc():
    a = MyAsyncDict()
    print(await a['x'])


if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(myfunc())
    finally:
        loop.close()

Simplified version:

import asyncio
import random


class MyAsyncDict:
    async def async_getitem(self, key):
        await asyncio.sleep(0.5)
        if random.random() < 0.5:
            return "value"
        else:
            raise KeyError(key)

    def __getitem__(self, key):
        return asyncio.create_task(self.async_getitem(key))


async def main():
    a = MyAsyncDict()
    print(f"Get {a['key']!r}")


asyncio.run(main())

Unfortunately __setitem__ is complicated as it doesn't return anything. The only workaround I found was with nest-asyncio, which monkey-patches asyncio to make it reentrant.

import asyncio
import random

import nest_asyncio

nest_asyncio.apply()


class MyAsyncDict:
    async def async_getitem(self, key):
        await asyncio.sleep(0.5)
        if random.random() < 0.5:
            return "value"
        else:
            raise KeyError(key)

    async def async_setitem(self, key, value):
        await asyncio.sleep(0.5)
        if random.random() < 0.5:
            raise KeyError(key)
        else:
            print(f"Set {key!r} to {value!r}")

    def __getitem__(self, key):
        return asyncio.run(self.async_getitem(key))

    def __setitem__(self, key, value):
        return asyncio.run(self.async_setitem(key, value))


async def main():
    a = MyAsyncDict()
    print(f"Get {a['key']!r} for key 'key'")
    a["key"] = "value"


asyncio.run(main())