What is the difference between async def and a regular function, and what does await actually do?
async def returns a coroutine; await suspends it until the awaited future completes
Calling an async def function does not run the body; it returns a coroutine object. The body runs when the coroutine is awaited or scheduled on an event loop. await takes an awaitable, such as a coroutine, a Task, or a Future, and suspends the current coroutine until that awaitable completes, then resumes it with the result or raises its exception. Suspension only happens if the awaited object is not already done; if it is done, await returns immediately without yielding to the loop. This is why await is a suspension point, not a thread switch: control returns to the event loop, which then runs other ready tasks. You can only use await inside async def, and async generators use async for and async with instead.
- 1
Regular def: runs immediately and returns the result. async def: returns a coroutine, runs when awaited or scheduled.
- 2
await obj: suspend until obj completes. Works with coroutines, Futures, and any object implementing await.
- 3
asyncio.create_task schedules a coroutine concurrently; await runs it as part of the current flow.
- 4
asyncio.gather runs awaitables concurrently and returns results in order.
- 5
Trade-off: async functions compose only with async callers. Mixing sync code requires explicit offloading via to_thread or executors.
- 6
Common mistake: calling an async function without await and getting a coroutine object that never runs, plus a RuntimeWarning.
- 7
Common mistake: assuming await creates concurrency. Awaiting sequentially runs tasks one at a time; you need create_task or gather.
- 8
Version note: the RuntimeWarning for never-awaited coroutines has been in 3.x for years. asyncio.to_thread was added in 3.9.
Scenario Questions
0-2 years experience
- 1You call an async function and print the result. Why do you see a coroutine object instead of the value?
- 2Where can you use await?
2-5 years experience
- 1You await three coroutines in a row and see no concurrency. What do you change?
- 2You need to run a coroutine in the background and not block the current flow. What do you use?
5-8 years experience
- 1You need to cancel a long-running coroutine when the client disconnects. How do you structure this with tasks and cancellation?
- 2You have a coroutine that returns a value and one that raises. How do you handle both with gather and return_exceptions?
8+ years experience
- 1Design a coroutine-based pipeline that supports back-pressure, cancellation, and per-task timeouts with predictable resource usage.
- 2Explain how Task, Future, and coroutine differ internally, and how exceptions propagate through await, gather, and TaskGroup.
Follow-up Questions
- What is the difference between awaiting two coroutines sequentially versus gathering them?
- How does an async generator differ from a regular generator?