asyncio and the Event Loop
A Python service that talks to many sockets at once, such as a gateway to several brokers or a monitor polling a set of venues, is often written with asyncio. The interview questions about it are consistent: how it differs from threads, what happens when a coroutine calls a blocking function, and where races can still happen when only one thread runs.
One thread, many waits
The event loop runs on a single thread. A coroutine runs until it reaches an await on something that is not ready, such as a socket read. It then hands control back to the loop, and the loop resumes another coroutine whose wait has finished. This is concurrency without parallelism: many operations are in progress, but only one line of Python runs at any moment.
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