Fluent Python

Chapter 18. Concurrency with asyncio

Concurrency is about dealing with lots of things at once.

Parallelism is about doing lots of things at once.

Not the same, but related.

One is about structure, one is about execution.

Concurrency provides a way to structure a solution to solve a problem that may (but not necessarily) be parallelizable.¹

Rob Pike, Co-inventor of the Go language

Professor Imre Simon² liked to say there are two major sins in science: using different words to mean the same thing and using one word to mean different things. If you do any research on concurrent or parallel programming you will find different definitions for “concurrency” and “parallelism.” I will adopt the informal definitions by Rob Pike, quoted above.

For real parallelism, you must have multiple cores. A modern laptop has four CPU cores but is routinely running more than 100 processes at any given time under normal, casual use. So, in practice, most processing happens concurrently and not in parallel. The computer is constantly dealing with 100+ processes, making sure each has an opportunity to make progress, even if the CPU itself can’t do more than four things at once. Ten years ago we used machines that were also able to handle 100 processes concurrently, but on a single core. That’s why Rob Pike titled that talk “Concurrency Is Not Parallelism (It’s Better).”

This chapter introduces asyncio, a package that implements concurrency with coroutines driven by an event loop. It’s one of the largest ...