Green Tea Press Think Python, How to Think Like a Computer Scientist (2008).pdf

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Think Python
How to Think Like a Computer Scientist
Version 1.1.19
Allen Downey
Green Tea Press
Needham, Massachusetts
Copyright © 2008 Allen Downey.
Printing history:
April 2002: First edition of How to Think Like a Computer Scientist .
August 2007: Major revision, changed title to How to Think Like a (Python) Programmer .
June 2008: Major revision, changed title to Think Python: How to Think Like a Computer Scientist .
Green Tea Press
9 Washburn Ave
Needham MA 02492
Permission is granted to copy, distribute, and/or modify this document under the terms of the GNU Free Doc-
umentation License, Version 1.1 or any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and with no Back-Cover Texts.
The GNU Free Documentation License is available from www.gnu.org or by writing to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
The original form of this book is L A T E X source code. Compiling this L A T E X source has the effect of generating
a device-independent representation of a textbook, which can be converted to other formats and printed.
The L A T E X source for this book is available from http://www.thinkpython.com
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 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.
• 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 students practice.
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.
Jeff and I revised the book, incorporated a case study by Chris Meyers, and in 2001 we released
How to Think Like a Computer Scientist: Learning with Python , also under the GNU Free Doc-
umentation License. As Green Tea Press, I published the book and started selling hard copies
through Amazon.com and college book stores. Other books from Green Tea Press are available at
greenteapress.com .
vi
Chapter 0. Preface
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.
Over the last five years I have continued to develop the book, correcting errors, improving some of
the examples and adding material, especially exercises. In 2008 I started work on a major revision—
at the same time, I was contacted by an editor at Cambridge University Press who was interested in
publishing the next edition. Good timing!
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 removed the material in the last few chapters about the implementation of lists and trees. I
still love those topics, but I thought they were incongruent with the rest of the book.
• I added more exercises, ranging from short tests of understanding to a few substantial projects.
• I added a series of case studies—longer examples with exercises, solutions, and discussion.
Some of them are based on Swampy, a suite of Python programs I wrote for use in my classes.
Swampy, code examples, and some solutions are available from thinkpython.com .
• I expanded the discussion of program development plans and basic design patterns.
• The use of Python is more idiomatic. The book is still about programming, not Python, but
now I think the book gets more leverage from the language.
I hope you enjoy working with this book, and that it helps you learn to program and think, at least a
little bit, like a computer scientist.
Allen B. Downey
Needham MA
Allen Downey is an Associate Professor of Computer Science at the Franklin W. Olin College of
Engineering.
Acknowledgements
First and most importantly, I thank 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 favorite language.
I also thank Chris Meyers, who contributed several sections to How to Think Like a Computer
Scientist .
And I thank the Free Software Foundation for developing the GNU Free Documentation License,
which helped make my collaboration with Jeff and Chris possible.
I also thank the editors at Lulu who worked on How to Think Like a Computer Scientist .
I thank all the students who worked with earlier versions of this book and all the contributors (listed
below) who sent in corrections and suggestions.
And I thank my wife, Lisa, for her work on this book, and Green Tea Press, and everything else, too.
 
vii
Contributor List
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 huge help.
If you have a suggestion or correction, please send email to feedback@thinkpython.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. Thanks!
• 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 “subcon-
sciously”.
• 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.
 

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