Evgenii Klimenko’s Post

How asyncio event loop actually works under the hood Async in Python often feels like magic. You write: await some_io() …and suddenly your app handles thousands of requests. But what’s really happening? At the core: event loop At the core of asyncio is the event loop. Think of it as a scheduler that constantly: • checks for ready tasks • runs them • pauses on await • switches to another task Key idea Async functions don’t run in parallel. They run cooperatively. Example: async def task():   await something() When Python hits await: • pauses the coroutine • returns control to the loop • runs another task Threads vs asyncio Threads: Thread A → running   Thread B → waiting Asyncio: Task A → waiting for I/O   Task B → running   Task C → ready What happens under the hood Very simplified loop: while True:   run ready tasks   wait for I/O The real magic — I/O When you do: await socket.recv() Python: • registers the socket in OS (epoll / kqueue) • pauses the coroutine • resumes it when data is ready No blocking. No busy waiting. Why it’s powerful • thousands of connections in one thread • low memory usage • no thread switching overhead Important limitation CPU-bound code blocks everything. for i in range(10_000_000):   pass → event loop freezes Rule of thumb I/O-bound → asyncio CPU-bound → multiprocessing Mental model Tasks → Event Loop → OS → Event Loop → Tasks Question Do you use asyncio in production or still prefer threads? #Python #AsyncIO #BackendDevelopment #SoftwareEngineering

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