Chapter 0  Preface

The strange history of this book

In January 1999 I was preparing to teach an introductory programming class in Java. I had taught it three times and I was getting frustrated. The failure rate in the class was too high and even for students who succeeded, the overall level of achievement was too low.

One of the problems I saw was the books. I had tried three different books (and read a dozen more), and they all had the same problems. They were too big, with too much unnecessary detail about Java, and not enough high-level guidance about how to program. And they all suffered from the trap door effect: they would start out very gradual and easy, and then somewhere around Chapter 5, the bottom would fall out. The students would get too much new material, too fast, and I would spend the rest of the semester picking up the pieces.

Two weeks before the first day of classes, I decided to write my own book. I wrote one 10-page chapter a day for 13 days. I made some revisions on Day 14 and then sent it out to be photocopied.

My goals were:

• Keep it short. It is better for students to read 10 pages than not read 50 pages.
• Be careful with vocabulary. I tried to minimize the jargon and define each term at first use.
• Build gradually. To avoid trap doors, I took the most difficult topics and split them into a series of small steps.
• It's not about the language; it's about programming. I included the minumum useful subset of Java and left out the rest.

I needed a title, so on a whim I chose How to Think Like a Computer Scientist.

My first version was rough, but it worked. Students did the reading, and they understood enough that I could spend class time on the hard topics, the interesting topics and (most important) letting the students practice.

As a user and advocate of free software, I believe in the idea Benjamin Franklin expressed:

“As we enjoy great Advantages from the Inventions of others, we should be glad of an Opportunity to serve others by any Invention of ours, and this we should do freely and generously.”

So I released the book under the GNU Free Documenation License, which allows users to copy, modify, and distribute the book.

What happened next is the cool part. Jeff Elkner, a high school teacher in Virginia, adopted my book and translated it into Python. He sent me a copy of his translation, and I had the unusual experience of learning Python by reading my own book.

Jeff and I revised the book, incorporated a case study by Chris Meyers, and released How to Think Like a Computer Scientist: Learning with Python, also under the GNU Free Documenation License.

At the same time, my wife and I started Green Tea Press, which distributes several of my books electronically, and sells How to Think in hard copy.

I have been teaching with this book for more than five years now, and I have done a lot more Python programming. I still like the structure of the book, but for some time I have felt the need to make changes:

• Some of the examples in the first edition work better than others. In my classes I have discarded the less effective ones and developed improvements.
• There are only a few exercises in the first edition. Now I have five years of quizzes, exams and homeworks to choose from.
• I have been programming in Python for a while now and have a better appreciation of idiomatic Python. The book is still about programming, not Python, but now I think the book gets more leverage from the language.

At the same time, Jeff has been working on his own second edition, customized for his classes. Rather than cram everything into one book (which may be how other books got so big), we decided to work on different versions. They are both under the Free Documentation License, so users can choose one or combine material from both.

For my version, I am using the revised title How to Think Like a (Python) Programmer. This is a more modest goal than the original, but it might be more accurate.

Allen B. Downey
Needham MA
Allen Downey is a Professor of Computer Science at the Franklin W. Olin College of Engineering.

For the teacher

Swampy and UML

For the student

Try out examples.

Do the in-chapter examples.

Where to get the code.

Contributor List

To paraphrase the philosophy of the Free Software Foundation, this book is free like free speech, but not necessarily free like free pizza. It came about because of a collaboration that would not have been possible without the GNU Free Documentation License. So we thank the Free Software Foundation for developing this license and, of course, making it available to us.

We also thank the more than 100 sharp-eyed and thoughtful readers who have sent us suggestions and corrections over the past few years. In the spirit of free software, we decided to express our gratitude in the form of a contributor list. Unfortunately, this list is not complete, but we are doing our best to keep it up to date.

If you have a chance to look through the list, you should realize that each person here has spared you and all subsequent readers from the confusion of a technical error or a less-than-transparent explanation, just by sending us a note.

Impossible as it may seem after so many corrections, there may still be errors in this book. If you should stumble across one, please check the online version of the book at http://thinkpython.com, which is the most up-to-date version. If the error has not been corrected, please take a minute to send us email at feedback@thinkpython.com. If we make a change due to your suggestion, you will appear in the next version of the contributor list (unless you ask to be omitted). Thank you!

• Lloyd Hugh Allen sent in a correction to Section 8.4.
• Yvon Boulianne sent in a correction of a semantic error in Chapter 5.
• Fred Bremmer submitted a correction in Section 2.1.
• Jonah Cohen wrote the Perl scripts to convert the LaTeX source for this book into beautiful HTML.
• Michael Conlon sent in a grammar correction in Chapter 2 and an improvement in style in Chapter 1, and he initiated discussion on the technical aspects of interpreters.
• Benoit Girard sent in a correction to a humorous mistake in Section 5.6.
• Courtney Gleason and Katherine Smith wrote horsebet.py, which was used as a case study in an earlier version of the book. Their program can now be found on the website.
• Lee Harr submitted more corrections than we have room to list here, and indeed he should be listed as one of the principal editors of the text.
• James Kaylin is a student using the text. He has submitted numerous corrections.
• David Kershaw fixed the broken catTwice function in Section 3.10.
• Eddie Lam has sent in numerous corrections to Chapters 1, 2, and 3. He also fixed the Makefile so that it creates an index the first time it is run and helped us set up a versioning scheme.
• Man-Yong Lee sent in a correction to the example code in Section 2.4.
• David Mayo pointed out that the word “unconsciously" in Chapter 1 needed to be changed to “subconsciously".
• Chris McAloon sent in several corrections to Sections 3.9 and 3.10.
• Matthew J. Moelter has been a long-time contributor who sent in numerous corrections and suggestions to the book.
• Simon Dicon Montford reported a missing function definition and several typos in Chapter 3. He also found errors in the increment function in Chapter 13.
• John Ouzts corrected the definition of “return value" in Chapter 3.
• Kevin Parks sent in valuable comments and suggestions as to how to improve the distribution of the book.
• David Pool sent in a typo in the glossary of Chapter 1, as well as kind words of encouragement.
• Michael Schmitt sent in a correction to the chapter on files and exceptions.
• Robin Shaw pointed out an error in Section 13.1, where the printTime function was used in an example without being defined.
• Paul Sleigh found an error in Chapter 7 and a bug in Jonah Cohen's Perl script that generates HTML from LaTeX.
• Craig T. Snydal is testing the text in a course at Drew University. He has contributed several valuable suggestions and corrections.
• Ian Thomas and his students are using the text in a programming course. They are the first ones to test the chapters in the latter half of the book, and they have made numerous corrections and suggestions.
• Keith Verheyden sent in a correction in Chapter 3.
• Peter Winstanley let us know about a longstanding error in our Latin in Chapter 3.
• Chris Wrobel made corrections to the code in the chapter on file I/O and exceptions.
• Moshe Zadka has made invaluable contributions to this project. In addition to writing the first draft of the chapter on Dictionaries, he provided continual guidance in the early stages of the book.
• Christoph Zwerschke sent several corrections and pedagogic suggestions, and explained the difference between gleich and selbe.
• James Mayer sent us a whole slew of spelling and typographical errors, including two in the contributor list.
• Hayden McAfee caught a potentially confusing inconsistency between two examples.
• Angel Arnal is part of an international team of translators working on the Spanish version of the text. He has also found several errors in the English version.
• Tauhidul Hoque and Lex Berezhny created the illustrations in Chapter 1 and improved many of the other illustrations.
• Dr. Michele Alzetta caught an error in Chapter 8 and sent some interesting pedagogic comments and suggestions about Fibonacci and Old Maid.
• Andy Mitchell caught a typo in Chapter 1 and a broken example in Chapter 2.
• Kalin Harvey suggested a clarification in Chapter 7 and caught some typos.
• Christopher P. Smith caught several typos and is helping us prepare to update the book for Python 2.2.
• David Hutchins caught a typo in the Foreword.
• Gregor Lingl is teaching Python at a high school in Vienna, Austria. He is working on a German translation of the book, and he caught a couple of bad errors in Chapter 5.
• Julie Peters caught a typo in the Preface.
• Florin Oprina sent in an improvement in makeTime, a correction in printTime, and a nice typo.
• D. J. Webre suggested a clarification in Chapter 3.
• Ken found a fistful of errors in Chapters 8, 9 and 11.
• Ivo Wever caught a typo in Chapter 5 and suggested a clarification in Chapter 3.
• Curtis Yanko suggested a clarification in Chapter 2.
• Ben Logan sent in a number of typos and problems with translating the book into HTML.
• Jason Armstrong saw the missing word in Chapter 2.
• Louis Cordier noticed a spot in Chapter 16 where the code didn't match the text.
• Brian Cain suggested several clarifications in Chapters 2 and 3.
• Rob Black sent in a passel of corrections, including some changes for Python 2.2.
• Jean-Philippe Rey at Ecole Centrale Paris sent a number of patches, including some updates for Python 2.2 and other thoughtful improvements.
• Jason Mader at George Washington University made a number of useful suggestions and corrections.
• Jan Gundtofte-Bruun reminded us that “a error” is an error.
• Abel David and Alexis Dinno reminded us that the plural of “matrix” is “matrices”, not “matrixes”. This error was in the book for years, but two readers with the same initials reported it on the same day. Weird.
• Charles Thayer encouraged us to get rid of the semi-colons we had put at the ends of some statements and to clean up our use of “argument” and “parameter”.
• Roger Sperberg pointed out a twisted piece of logic in Chapter 3.
• Sam Bull pointed out a confusing paragraph in Chapter 2.
• Andrew Cheung pointed out two instances of “use before def.”
• C. Corey Capel spotted the missing word in the Third Theorem of Debugging.