notes.md

Chapter 21: Asynchronous Programming — Notes

1. The Event Loop Architecture

asyncio does not use multiple threads to achieve concurrency. It uses exactly one thread and an Event Loop. The event loop acts like a highly efficient traffic cop. It tracks multiple tasks. When Task A needs to wait for a network request, it explicitly yields control back to the event loop using the await keyword. The event loop instantly switches execution to Task B.

2. Terminology

• Coroutine Function: A function defined with async def. When called, it does not execute. Instead, it returns a Coroutine Object.
• Coroutine Object: The object returned by an async def function. It must be driven by the event loop, usually via await.
• Awaitable: Any object that can be used with the await keyword. This includes Coroutines, Tasks, and Futures.

3. The asyncio Core APIs

• asyncio.run(coro): The entry point. It creates a brand new event loop, runs the main coroutine until it finishes, and cleanly shuts the loop down.
• await: Pauses the current coroutine, allowing the event loop to switch to other pending tasks until the awaited operation finishes.
• asyncio.gather(*awaitables): Schedules multiple tasks to run concurrently. It pauses the current coroutine until all of them are finished, then returns their results as a list in the exact order they were passed in.
• asyncio.create_task(coro): Schedules a coroutine to run on the event loop immediately "in the background", returning a Task object. You don't have to await it immediately.

4. The Golden Rule of Async

Never block the event loop. Because there is only one thread, if you execute a blocking synchronous function (like requests.get(), time.sleep(), or massive mathematical loops) inside a coroutine, the thread halts. The event loop freezes. Zero other tasks can run. Every single connected client to your server will hang. If you must use blocking code, you must offload it using asyncio.to_thread().