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This is an intermediate-to-advanced book on concurrency in Python.

The reader is assumed to already be fluent in the Python language. There are a large number and variety of resources available to achieve this fluency, many of them free.

In addition, I use the latest version of Python available at this writing, version 3.6, which includes important concurrency features. If you are wedded to an earlier version of Python, this book might not be for you.

That said, this book is an introduction to the concepts and applications of concurrency in Python, for a beginner to those topics. I start from scratch, assuming you've never heard the term before or any of the surrounding ideas. This is a very reasonable approach because concurrency is relatively orthogonal to the rest of programming and it's entirely possible to be an expert in everything about a language for years before exploring concurrency for the first time.

A Strange but Pragmatic Approach

Virtually every discussion of concurrency starts with the complex, low-level nuts and bolts and then slowly works up to the more elegant high-level solutions. This is understandable because it's how we teach everything else.

Concurrency is a strange topic in just about every conceivable way. As you shall learn in this book, the only justification for wading into the complexity and tribulations of concurrency is to make your program run faster. And if that's truly what you need, you probably don't need to learn everything about concurrency. What you'd really like is the shortest, easiest path to a faster program.

So that's how I organize the book. After explaining what concurrency is, we'll start with the simplest and easiest---and typically, the highest level---approaches to speeding up your program with concurrency. As the book progresses, the techniques will become lower-level, messier, and more complicated. The goal is that you'll go only as far as you need in order to solve your speed problem, and after that you'll only go further if you're interested, or when you come back later with a new problem.

eBook First

This book is developed in the open, as an eBook. Although I haven't yet settled on a license, my intent is for the eBook to remain free.

At the 2016 Pycon in Portland, I held an open-spaces session around this book, and also attended a dinner where we talked about it. In both cases people argued that, while they really liked the idea of a free eBook, eventually they want a print book. Once the eBook version has been out long enough to evolve and settle down, and also to get adequate feedback to eliminate errors, I will revisit the issue and consider creating a print version of the book.

Business Model

My two most recent books, Atomic Scala and On Java 8, were not free. In the process of developing those books I became aware that a big part of that choice was that---while I enjoyed learning about those languages and writing those books---I ultimately didn't want to do further work in those areas, so there was no additional financial support (seminars, consulting, etc.) created from those books. They needed to be their own endpoints.

The Python community has called to me ever since I discovered it. I have postponed that call for a couple of decades, with the excuse "let me just finish this one thing, then I'll come join you" (an all-too-familiar refrain heard by the partners of computer programmers). I evolve slowly, but I know now that the Python community is where I'm happiest.

While the thought of struggling with the arbitrary limitations and roadblocks of, say, the Java language (for details, see On Java 8) in a training or consulting context fills me with angst, the vision of working with people who already know the productivity and delights of Python fills me with joy. That's where I want to live, and I want this book to lay the groundwork for conferences, developer retreats, consulting, training, and other experiences. I believe it will satisfy my need for fun and fulfillment.

I could certainly have taken the approach I've used in my other books and created an introductory Python book, but there are a vast number of those which do an outstanding job. I don't see myself making a big contribution there. Instead, I chose something I think most folks experience as quite challenging. Over the decades, I have had periodic, intense bouts with concurrency. I've come away from each of these believing that, finally, I understand the topic. And each time I've discovered later there was some gaping hole in my knowledge. You will discover that concurrency consistently produces excellent examples of the Dunning-Kruger Effect, where the less you know, the more you think you know. I know enough about concurrency to know there can always appear some surprising hole in my knowlege.

Source Code

All the source code for this book is available as copyrighted freeware, distributed along with the sources for the book's text, via Github. To make sure you get the most current version, this is the official code distribution site.

Coding Standards

In the text of this book, identifiers (keywords, methods, variables, and class names) are set in bold, fixed-width code font. Some keywords, such as class, are used so much that the bolding can become tedious. Those which are distinctive enough are left in normal font.

The code files in this book are tested with an automated system, and should work without compiler errors (except those specifically tagged) in the latest version of Python.

Bug Reports

No matter how many tools a writer uses to detect errors, some always creep in and these often leap off the page for a fresh reader. If you discover anything you believe to be an error, please submit the error along with your suggested correction, for either the book's prose or examples, here. Your help is appreciated.

Mailing List

For news and notifications, you can subscribe to the low-volume email list at I don't use ads and strive to make the content as appropriate as possible.


This book was written with Pandoc-flavored Markdown, and produced into ePub version 3 format using Pandoc.

The body font is Georgia and the headline font is Verdana. The code font is Ubuntu Mono, because it is especially compact and allows more characters on a line without wrapping. I chose to place the code inline (rather than make listings into images, as I've seen some books do) because it is important to me that the reader be able to resize the font of the code listings when they resize the body font (otherwise, really, what's the point?).

The build process for the book is automated, as well as the process to extract, compile and test the code examples. All automation is achieved through fairly extensive programs I wrote in Python 3.

Cover Design