[Python] Why CPU bound task doesn't block asyncio event loop?

I want to go deep in asynchronous task execution in python and I made experiment when CPU Bound task runs with IO bound task simultaneously.

I suppose that CPU Bound task blocks the event loop and IO Bound task will start after CPU Bound task finished. But in my experiment I see that IO Bound task start and finishes simultaneously with CPU Bound. I want to find answer, why it happens.

To make IO bound load I use asyncio.sleep, to make CPU bound load I use for loop with multiplying and division.

Experiment code:

import asyncio
from datetime import datetime

async def io_bound(i):
print(f'start io_bound {i} {datetime.now().strftime("%H:%M:%S")}')
await asyncio.sleep(1)
print(f'finish io_bound {i} {datetime.now().strftime("%H:%M:%S")}')

async def cpu_bound():
print(f'start cpu_bound {datetime.now().strftime("%H:%M:%S")}')
for i in range(5 * 10**7):
i *= i+1
i /= i+1
print(f'finish cpu_bound {datetime.now().strftime("%H:%M:%S")}')

async def main():
await io_bound(1)
asyncio.create_task(cpu_bound())
await io_bound(2)

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
loop.run_until_complete(main())


output:

start io_bound 1 17:28:20
finish io_bound 1 17:28:21
start io_bound 2 17:28:21
start cpu_bound 17:28:21
finish cpu_bound 17:28:26
finish io_bound 2 17:28:26


I expected that output should be:

start io_bound 1 17:28:20
finish io_bound 1 17:28:21
start io_bound 2 17:28:21
start cpu_bound 17:28:21
finish cpu_bound 17:28:26
finish io_bound 2 17:28:27


That is, io_bound task should finish one second after cpu_bound is finished.

So the question is: Why io_bound finishes immediately after cpu_bound is finished? Doesn't CPU bound task block execution of other tasks?

Thanks for any responses.

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