Here’s the story: last week my lead asked me to optimize a data aggregation service that calls 20 downstream APIs. The serial version took around 18 seconds — users were ready to throw their keyboards. Obvious IO-bound job, right? I thought I’d slap on asyncio, ship it in half a day, and look like a hero. Instead, I spent three hours falling into every rabbit hole asyncio had to offer, and nearly took down production. This post walks through the three biggest pitfalls I hit and how to write async code that actually works in the real world.

Get Your Concepts Straight First

At its core, asyncio is a single-threaded event loop — a master scheduler that lines up coroutines. When one coroutine is waiting on IO, the loop politely tells it to step aside and runs whichever coroutine is ready instead.…