Deferred future for controlling async flow (there are existing asyncio features for doing this)

deferred_future.py

import asyncio
import json

from typing import TypeVar, Generic, List

T = TypeVar("T")


class DeferredFuture(Generic[T]):
    def __init__(self):
        self.future = asyncio.Future[T]()

    def __call__(self, result: T):
        if not self.future.done():
            self.future.set_result(result)

    def __await__(self):
        result = yield from self.future.__await__()
        return result

    def reject(self, reason: Exception):
        if not self.future.done():
            self.future.set_exception(reason)