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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.
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 easy,
proceed gradually, 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. 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 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.
- Focus on programming, not the programming language. I included
the minimum 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
I released the book under the GNU Free Documentation 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.
As Green Tea Press, I published the first Python version in 2001.
In 2003 I started teaching at Olin College and I got to teach
Python for the first time. The contrast with Java was striking.
Students struggled less, learned more, worked on more interesting
projects, and generally had a lot more fun.
Since then I’ve continued to develop the book,
correcting errors, improving some of the examples and
adding material, especially exercises.
The result is this book, now with the less grandiose title
Think Python. Some of the changes are:
- I added a section about debugging at the end of each chapter.
These sections present general techniques for finding and avoiding
bugs, and warnings about Python pitfalls.
- I added more exercises, ranging from short tests of
understanding to a few substantial projects. Most exercises
include a link to my solution.
- I added a series of case studies—longer examples with
exercises, solutions, and discussion.
- I expanded the discussion of program development plans
and basic design patterns.
- I added appendices about debugging and analysis of algorithms.
The second edition of Think Python has these new features:
- The book and all supporting code have been updated to Python 3.
- I added a few sections, and more details on the web, to help
beginners get started running Python in a browser, so you don’t have
to deal with installing Python until you want to.
- For Chapter 4.1 I switched from my own turtle graphics
package, called Swampy, to a more standard Python module, turtle, which is easier to install and more powerful.
- I added a new chapter called “The Goodies”, which introduces
some additional Python features that are not strictly necessary, but
I hope you enjoy working with this book, and that it helps
you learn to program and think like
a computer scientist, at least a little bit.
Allen B. Downey
Many thanks to Jeff Elkner, who
translated my Java book into Python, which got this project
started and introduced me to what has turned out to be my
Thanks also to Chris Meyers, who contributed several sections
to How to Think Like a Computer Scientist.
Thanks to the Free Software Foundation for developing
the GNU Free Documentation License, which helped make
my collaboration with Jeff and Chris possible, and Creative
Commons for the license I am using now.
Thanks to the editors at Lulu who worked on
How to Think Like a Computer Scientist.
Thanks to the editors at O’Reilly Media who worked on
Thanks to all the students who worked with earlier
versions of this book and all the contributors (listed
below) who sent in corrections and suggestions.
More than 100 sharp-eyed and thoughtful readers have sent in
suggestions and corrections over the past few years. Their
contributions, and enthusiasm for this project, have been a
If you have a suggestion or correction, please send email to
email@example.com. If I make a change based on your
feedback, I will add you to the contributor list
(unless you ask to be omitted).
If you include at least part of the sentence the
error appears in, that makes it easy for me to search. Page and
section numbers are fine, too, but not quite as easy to work with.
- Lloyd Hugh Allen sent in a correction to Section 8.4.
- Yvon Boulianne sent in a correction of a semantic error in
- 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.
- Benoît 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
- David Kershaw fixed the broken catTwice function in Section
- 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
- 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
- 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
- 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
- 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 helped us
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 École 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 and a typo in Chapter 4.
- Alessandra helped clear up some Turtle confusion.
- Wim Champagne found a brain-o in a dictionary example.
- Douglas Wright pointed out a problem with floor division in
- Jared Spindor found some jetsam at the end of a sentence.
- Lin Peiheng sent a number of very helpful suggestions.
- Ray Hagtvedt sent in two errors and a not-quite-error.
- Torsten Hübsch pointed out an inconsistency in Swampy.
- Inga Petuhhov corrected an example in Chapter 14.
- Arne Babenhauserheide sent several helpful corrections.
- Mark E. Casida is is good at spotting repeated words.
- Scott Tyler filled in a that was missing. And then sent in
a heap of corrections.
- Gordon Shephard sent in several corrections, all in separate
- Andrew Turner spotted an error in Chapter 8.
- Adam Hobart fixed a problem with floor division in arc.
- Daryl Hammond and Sarah Zimmerman pointed out that I served
up math.pi too early. And Zim spotted a typo.
- George Sass found a bug in a Debugging section.
- Brian Bingham suggested Exercise 5.
- Leah Engelbert-Fenton pointed out that I used tuple
as a variable name, contrary to my own advice. And then found
a bunch of typos and a “use before def”.
- Joe Funke spotted a typo.
- Chao-chao Chen found an inconsistency in the Fibonacci example.
- Jeff Paine knows the difference between space and spam.
- Lubos Pintes sent in a typo.
- Gregg Lind and Abigail Heithoff suggested Exercise 3.
- Max Hailperin has sent in a number of corrections and
suggestions. Max is one of the authors of the extraordinary Concrete Abstractions, which you might want to read when you are
done with this book.
- Chotipat Pornavalai found an error in an error message.
- Stanislaw Antol sent a list of very helpful suggestions.
- Eric Pashman sent a number of corrections for Chapters 4–11.
- Miguel Azevedo found some typos.
- Jianhua Liu sent in a long list of corrections.
- Nick King found a missing word.
- Martin Zuther sent a long list of suggestions.
- Adam Zimmerman found an inconsistency in my instance
of an “instance” and several other errors.
- Ratnakar Tiwari suggested a footnote explaining degenerate
- Anurag Goel suggested another solution for
and sent some additional corrections. And he knows how to
spell Jane Austen.
- Kelli Kratzer spotted one of the typos.
- Mark Griffiths pointed out a confusing example in Chapter 3.
- Roydan Ongie found an error in my Newton’s method.
- Patryk Wolowiec helped me with a problem in the HTML version.
- Mark Chonofsky told me about a new keyword in Python 3.
- Russell Coleman helped me with my geometry.
- Nam Nguyen found a typo and pointed out that I used the Decorator
pattern but didn’t mention it by name.
- Stéphane Morin sent in several corrections and suggestions.
- Paul Stoop corrected a typo in
- Eric Bronner pointed out a confusion in the discussion of the
order of operations.
- Alexandros Gezerlis set a new standard for the number and
quality of suggestions he submitted. We are deeply grateful!
- Gray Thomas knows his right from his left.
- Giovanni Escobar Sosa sent a long list of corrections and
- Daniel Neilson corrected an error about the order of operations.
- Will McGinnis pointed out that polyline was defined
differently in two places.
- Frank Hecker pointed out an exercise that was under-specified, and
some broken links.
- Animesh B helped me clean up a confusing example.
- Martin Caspersen found two round-off errors.
- Gregor Ulm sent several corrections and suggestions.
- Dimitrios Tsirigkas suggested I clarify an exercise.
- Carlos Tafur sent a page of corrections and suggestions.
- Martin Nordsletten found a bug in an exercise solution.
- Sven Hoexter pointed out that a variable named input
shadows a build-in function.
- Stephen Gregory pointed out the problem with cmp
in Python 3.
- Ishwar Bhat corrected my statement of Fermat’s last theorem.
- Andrea Zanella translated the book into Italian, and sent a
number of corrections along the way.
- Many, many thanks to Melissa Lewis and Luciano Ramalho for
excellent comments and suggestions on the second edition.
- Thanks to Harry Percival from PythonAnywhere for his help
getting people started running Python in a browser.
- Xavier Van Aubel made several useful corrections in the second
- William Murray corrected my definition of floor division.
- Per Starbäck brought me up to date on universal newlines in Python 3.
- Laurent Rosenfeld and Mihaela Rotaru translated this book into French. Along the way, they sent many corrections and suggestions.
In addition, people who spotted typos or made corrections include
Czeslaw Czapla, Dale Wilson, Francesco Carlo Cimini,
Richard Fursa, Brian McGhie, Lokesh Kumar Makani, Matthew Shultz, Viet
Le, Victor Simeone, Lars O.D. Christensen, Swarup Sahoo, Alix Etienne,
Kuang He, Wei Huang, Karen Barber, and Eric Ransom.
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