Async

Async Await

async def creates coroutines, and await pauses until awaitable work completes.

An async def function returns a coroutine object when called. The function body has not produced its final result yet.

Source

import asyncio

async def fetch_title(slug):
    await asyncio.sleep(0)
    return slug.replace("-", " ").title()

coroutine = fetch_title("async-await")
print(coroutine.__class__.__name__)
coroutine.close()

Output

coroutine

Use await inside another coroutine to get the eventual result. asyncio.run() starts an event loop for the top-level coroutine.

Source

async def main():
    title = await fetch_title("async-await")
    print(title)

asyncio.run(main())

Output

Async Await

asyncio.gather() awaits several awaitables and returns their results in order. This is the shape used when independent I/O operations can progress together.

Source

async def main():
    titles = await asyncio.gather(fetch_title("json"), fetch_title("datetime"))
    print(titles)

asyncio.run(main())

Output

['Json', 'Datetime']

async with and async for are the asynchronous forms of context managers and iteration. A class implements __aenter__/__aexit__ to act as an async context manager; an async def function with yield becomes an async generator. The dedicated [async iteration and context](/iteration/async-iteration-and-context) page explains the protocols in depth.

Source

class Session:
    async def __aenter__(self):
        print("open")
        return self

    async def __aexit__(self, *_):
        print("close")
        return False


async def stream():
    for slug in ["json", "datetime"]:
        await asyncio.sleep(0)
        yield slug


async def driver():
    async with Session():
        async for slug in stream():
            print(slug)

asyncio.run(driver())

Output

open
json
datetime
close

Notes

Calling an async function creates a coroutine object.
await yields control until an awaitable completes.
Workers request handlers are async, so this pattern appears around fetches and bindings.
Prefer ordinary functions when there is no awaitable work to coordinate.

Run the complete example

Example code

Expected output

Async Await
['Json', 'Datetime']
open
json
datetime
close

Execution time appears here after you run the example.

Async Await

See also

Run the complete example

Example code

Expected output