Python course / Applied

Async syntax and event loops

Write coroutines that wait without blocking, and run many of them at once. Async is about waiting, not speed, and one blocking call freezes every task you started.

async and await
gather
tasks
timeouts
cancellation
TaskGroup
queues
semaphores

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three waits, one second

import asyncio
import time


async def fetch(name, seconds):
    await asyncio.sleep(seconds)
    return f"{name} after {seconds}s"


async def main():
    started = time.perf_counter()
    results = await asyncio.gather(
        fetch("tea", 1), fetch("jam", 1), fetch("oats", 1)
    )
    for line in results:
        print(line)
    print(f"elapsed: {time.perf_counter() - started:.1f}s")


asyncio.run(main())

tea after 1s
jam after 1s
oats after 1s
elapsed: 1.0s

What you will learn

Write a coroutine function with async def, wait inside it with await asyncio.sleep, and run it with asyncio.run.
See that awaits one after another take the sum of their waits, and time an awaitable with a helper.
Run several coroutines concurrently with asyncio.gather, and know its results come back in argument order.
Put a time limit on an await with asyncio.timeout, and know the work that ran out of time is cancelled.
Run tasks in an asyncio.TaskGroup, which waits for all of them and cancels the rest when one fails, and handle its errors with except*.
Cap how many coroutines do something at the same time with asyncio.Semaphore.

The lessons in this module

Waiting, not working
A coroutine is an object
Awaiting in sequence
gather: waiting on several at once
Tasks: starting work now
An event loop by hand
Blocking the loop
Timeouts
Cancellation
Errors in gather
TaskGroup: all or nothing
Queues: sharing out the work
Limiting how many at once
async with and async for
Capstone: a polite crawler
Module review: the order board

Ready when you are

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