Python async and await: Write Event Loop Code

Async Await

async and await let Python pause one task while waiting and continue another task in the same event loop.

Async code is useful for I/O-heavy work such as many network requests, sockets, or cooperative background tasks.

Do not use async just because it looks advanced. Use it when libraries and workload fit the event-loop model.

Async Function

An async def function returns a coroutine. It runs when awaited or passed into the event loop.

Use async def to define a coroutine function.
Use await inside async code to pause for another awaitable.
Use asyncio.run for a simple program entry point.

Run a Coroutine

import asyncio

async def greet():
    await asyncio.sleep(0)
    return "Hello async"

message = asyncio.run(greet())
print(message)

Output

Hello async

asyncio.run starts the event loop and returns the coroutine result.

Concurrent Awaits

asyncio.gather waits for multiple awaitable tasks and returns their results together.

Use gather when tasks can wait independently.
Keep blocking functions out of async code.
Use async libraries for real network work.

Gather Results

import asyncio

async def load(name):
    await asyncio.sleep(0)
    return name.upper()

async def main():
    results = await asyncio.gather(load("csv"), load("json"))
    print(results)

asyncio.run(main())

Output

['CSV', 'JSON']

Both coroutines are scheduled and gathered by the event loop.

Before you move on

Can You Use Async Await?

5 checks

You can explain coroutine, await, and event loop in plain language.
You can run one coroutine with asyncio.run.
You can await another async function.
You can gather multiple awaitables.
You can decide when async is more appropriate than normal sequential code.

Await Flow Traps

Calling async functions like normal functions
Await the coroutine or run it through asyncio.run.
Putting blocking work in async code
Use async-compatible libraries or keep blocking work outside the event loop.
Using async for every beginner script
Use normal functions until the program has real concurrent I/O needs.

Try this next

Await Slow Tasks

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Write two coroutines
Create load_user and load_orders coroutines that return strings, then gather both.
Explain await
Write one sentence explaining what happens when a coroutine reaches await.
Compare with threading
List one task that fits threading and one task that fits async.

Questions About Async Await

Only for the right kind of waiting-heavy workload. It does not automatically speed up ordinary calculations.

You usually get a coroutine object instead of the final result, and the intended work may not run.

Advanced programs can, but beginners should learn each model separately first.

Previous Next

Python async and await: Write Event Loop Code

Async Await

Async Function

Run a Coroutine

Concurrent Awaits

Gather Results

Can You Use Async Await?

Await Flow Traps

Calling async functions like normal functions

Putting blocking work in async code

Using async for every beginner script

Await Slow Tasks

Write two coroutines

Explain await

Compare with threading

Async Await Learning Path

Questions About Async Await

Is async faster than normal code?

What happens if I forget await?

Can I mix threads and async?

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