There is no sole or strict definition of clean code. Moreover, there is probably no way of formally measuring clean code, so you cannot run a tool on a repository that could tell you how good, bad, or maintainable or not that code is. Sure, you can run tools such as checkers, linters, static analyzers, and so on. And those tools are of much help. They are necessary, but not sufficient. Clean code is not something a machine or script could tell (so far), but rather something that us, as professionals, can decide.

For decades of using the terms programming languages, we thought that they were languages to communicate our ideas to the machine, so it can run our programs. We were wrong. That's not the truth, but part of the truth. The real language behind programming languages is to communicate our ideas to other developers.

Here is where the true nature of clean code lies. It depends on other engineers to be able to read and maintain the code. Therefore, we, as professionals, are the only ones who can judge this. Think about it; as developers, we spend much more time reading code than actually writing it. Every time we want to make a change or add a new feature, we first have to read all the surroundings of the code we have to modify or extend. The language (Python), is what we use to communicate among ourselves.

So, instead of giving you a definition (or my definition) of clean code, I invite you to go through the book, read all about idiomatic Python, see the difference between good and bad code, identify traits of good code and good architecture, and then come up with your own definition. After reading this book, you will be able to judge and analyze code for yourself, and you will have a more clear understanding of clean code. You will know what it is and what it means, regardless of any definition given to you.