Example for python 3.5 showing how example code can cause missed done messages. Contains also alternative example code.

test_asyncio.py

import unittest
import asyncio
import random


def generate_numbers():
    for index in range(100):
        yield index


async def producer(sequence):
    await asyncio.sleep(random.randrange(1, 101)/1000)
    try:
        return next(sequence)
    except StopIteration:
        return None


async def bad_example():
    got = set()
    missed = set()
    sequence = generate_numbers()
    while len(got) + len(missed) < 100:
        task_1 = asyncio.ensure_future(producer(sequence))
        task_2 = asyncio.ensure_future(producer(sequence))

        done, pending = await asyncio.wait([task_1, task_2], return_when=asyncio.FIRST_COMPLETED)

        if task_1 in done:
            value = task_1.result()
            if value is not None:
                got.add(value)
            await asyncio.sleep(0.1) # assume operations here can take a while
        else:
            if not task_1.cancel():
                value = task_1.result()
                if value is not None:
                    missed.add(value)

        if task_2 in done:
            value = task_2.result()
            if value is not None:
                got.add(value)
            await asyncio.sleep(0.1) # assume operations here can take a while
        else:
            if not task_2.cancel():
                value = task_2.result()
                if value is not None:
                    missed.add(value)
    return len(got), len(missed)


async def improved_example():
    got = set()
    sequence = generate_numbers()
    task_1 = asyncio.ensure_future(producer(sequence))
    task_2 = asyncio.ensure_future(producer(sequence))
    pending = {task_1, task_2}
    while pending:
        done, pending = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)

        if task_1 in done:
            value = task_1.result()
            if value is not None:
                got.add(value)
                await asyncio.sleep(0.1) # assume operations here can take a while
                task_1 = asyncio.ensure_future(producer(sequence))
                pending.add(task_1)

        if task_2 in done:
            value = task_2.result()
            if value is not None:
                got.add(value)
                await asyncio.sleep(0.1) # assume operations here can take a while
                task_2 = asyncio.ensure_future(producer(sequence))
                pending.add(task_2)
    return len(got)


class AsyncIOTestCase(unittest.TestCase):

    def test_bad_example(self):
        got, missed = asyncio.get_event_loop().run_until_complete(bad_example())
        self.assertEqual(100, got + missed)
        self.assertEqual(100, got, "Got is {}. Missed {}".format(got, missed))

    def test_improved_example(self):
        got = asyncio.get_event_loop().run_until_complete(improved_example())
        self.assertEqual(100, got)


if __name__ == "__main__":
    unittest.main()