Threads let the operating system switch between them at any moment. asyncio takes another route: one thread, and tasks that hand over control only at the points you mark with await.

import asyncio

async def size_of(name):
    await asyncio.sleep(0.01)  # pretend I/O
    return len(name)

async def main():
    a = await size_of("notes.txt")
    b = await size_of("todo.md")
    return a + b

print(asyncio.run(main()))  # 16

• async def defines a coroutine function. Calling it runs nothing: it returns a coroutine object.
• await runs a coroutine to its result. While it waits, the event loop can run other work.
• asyncio.run(main()) starts an event loop, runs main to the end and closes the loop. Call it once, from ordinary code.

await only works inside async def. Forgetting it is the classic mistake: you get a coroutine object instead of a value, and Python warns that the coroutine was never awaited.

Write async def total_size(names, size_of) that awaits size_of(name) for each name, in order, and returns the sum. Then write an ordinary function report(names, size_of) that runs it with asyncio.run and returns a string such as "16 bytes".

The explanation opens here once every check passes.