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Sun 20 November 2022

Book Review: CPython Internals

Posted by Simon Larsén in Reviews   

About a month ago I signed myself up to do a talk at a Python meetup hosted by HiQ. I brazenly set my topic as Under the Hood of CPython, thinking I had sufficient understanding of its inner workings to produce a riveting talk. As I started preparing the talk, I came to the gut-wrenching conclusion that my knowledge was too shallow, I simply didn't know enough of the details to put together an in-depth talk on the subject. Thankfully, I knew where to turn to for the details I needed: Anthony Shaw's CPython Internals book. Here's what I think of it.

CPython Internals
by Anthony Shaw
Released May 2021
Publisher(s): Real Python (
ISBN: 9781775093344

Confused about the difference between Python and CPython? See The difference between Python and CPython.

The book in a nutshell

CPython Internals gives you a guided tour of the CPython project, from parsing source code to compiling bytecode to interpreting said bytecode. The book is meant to serve as a starting point for budding CPython contributors or Python developers that simply want to learn a bit more about the reference implementation. It highlights the most important files in the project for each of the respective parts and guides you through their execution. Throughout the book we also get to follow along with a worked example of extending the language with an "almost equal" operator, written as ~=.

The book concludes with three concrete ways in which you can use the knowledge you've attained: 1) creating C extensions, 2) improving existing Python programs by leveraging knowledge of the internals and 3) contributing to the CPython project. While point 2) is potentially a little bit vague, points 1) and 3) are concrete and well described.

Before writing the book, Anthony wrote an in-depth article on the same topic. You can find it over on Real Python. It is something of an appetizer for the book, but stands strong on its own. Having a brief look at that article will give you a better understanding of what the book is about than anything I could write here.

What I liked

I went from surface-level understanding of the CPython project to being pretty confident about where to poke around to do what just from reading this book. It's comprehensive in scope and the worked example of the ~= operator helps a lot in facilitating an understanding for how to extend CPython with your own silly things.

I also appreciated the nods to other respectable sources. The Python Developer's Guide is a great resource for quickly refreshing how to do something (but going from 0 knowledge about the project it's a bit to terse). Luciano Ramalho's book Fluent Python 2nd ed is also noted as an excellent reference on the Python object model, which I absolutely agree with.

There is enough context in each chapter that you don't really need much pre-existing understanding of any of the subjects. If you can read Python code and have a little bit of experience with reading C code, you're all good to go. Concepts relevant to the book such as parallelism and memory management are explained both on an abstract level and in how they are implemented in CPython. The book is to a great extent a standalone resource and it should be very approachable even to developers without much experience. There's even an appendix at the end to explain what little you need to know about the C programming language to be able to understand the code samples.

What I didn't like

There was nothing about the book that I thought was bad, but for me personally, I would have preferred less explanation of fundamental concepts (such as threading), and more in-depth details on CPython itself. That being said, I think Anthony overall has made good calls on the tradeoffs between depth and approachability. Given that the book is meant to be a starting point for CPython development as opposed to a complete reference, I think that this nit-pick of mine is nothing more than personal preference, and perhaps that I'm slightly outside the target audience of the book. It's clearly meant to contain everything you'd need to know to go from zero to hero, and it's a lot easier to skip over content you feel is redundant than it is to find content you didn't know you needed.


CPython Internals saved my neck. I had three weeks to go from a shallow understanding of the CPython project to being able to explain it to others in a ~30-minute talk, and I made it. Without this book I wouldn't have. It was so approachable that I could use it for "Sunday reading" before bedtime and in spare minutes on public transport.

I highly recommend this book to anyone who's interested in the CPython project. It's not necessary to have future CPython contributions as a goal to get a lot out of this book, I found it incredibly interesting in its own right. The fact that Anthony has managed to pack so much information, with so much context (recall that I thought there was too much of that) in less than 400 pages is nothing short of spectacular.