Simple implementation showing the idea of async code. Doesn't use async/await keywords or real event_loop for the sake of simplicity and clarity

dummy_async.py

import time
from collections import deque
from pprint import pprint

START_TIME = time.time()


def run(func):
    gen = func()
    while True:
        try:
            next(gen)
        except StopIteration as e:
            return e.value


def gather(*tasks):
    results = [None] * len(tasks)
    working_tasks = deque(enumerate(tasks))

    while working_tasks:
        idx, task = working_tasks.popleft()
        try:
            next(task)
            working_tasks.append((idx, task))
        except StopIteration as e:
            results[idx] = e.value
        yield

    return results


def timer(seconds):
    start = time.time()
    while time.time() - start < seconds:
        yield


def async_request(name, delay: float):
    print(f"Request {name} started: {time.time() - START_TIME:.2f}")
    yield from timer(delay)
    print(f"Request {name} finished: {time.time() - START_TIME:.2f}")

    return f"Response from {name}: {time.time() - START_TIME:.2f}"


def complex_function():
    d = yield from async_request("D", 0.5)
    e = yield from async_request("E", 1)
    f = yield from gather(async_request("F1", 1.4), async_request("F2", 1.6))
    return [d, e, f]


def main():
    results = yield from gather(
        async_request("A", 3),
        async_request("B", 2),
        async_request("C", 1),
        complex_function(),
    )
    print("----------------")
    print("Results:")
    pprint(results)


if __name__ == "__main__":
    run(main)