学习Python编程：通往计算机科学家之路

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"《Python Programming》是一本专为计算机科学学生编写的大学教材，旨在帮助读者像计算机科学家一样思考，并通过Python3学习编程。本书由Peter Wentworth, Jeffrey Elkner, Allen B. Downey和Chris Meyers合著，涵盖了Python编程的基础到进阶知识，包括程序设计的基本概念、变量与表达式、语句以及使用turtle模块进行图形编程等内容。" 在本书中，作者首先介绍了“编程之道”，解释了Python编程语言的基本特性，并引导读者理解什么是程序、调试的重要性以及程序中的错误类型：语法错误、运行时错误和语义错误。书中还强调了实验性调试作为解决错误的一种方法，以及形式语言和自然语言之间的区别。此外，书中介绍了编写第一个程序的基本步骤，包括如何添加注释。接下来，作者深入讨论了变量、表达式和语句。值和数据类型是编程的基础，包括整数、浮点数、字符串等。变量用于存储这些值，而变量名的选择需遵循一定的规则，避免与Python的关键字冲突。语句是程序的基本构建块，表达式则是计算和操作的中心。书中还详细讲解了运算符和操作数、类型转换函数以及运算顺序。字符串操作包括拼接、切片等，同时，还介绍了输入和输出的基本方法。在程序结构方面，作者引入了组合（composition）的概念，如使用缩进来表示代码块，并通过实例介绍了模运算符的用法。此外，书中还详细讲解了for循环，它是控制程序执行流程的关键，使得可以对集合中的每个元素执行相同的操作。第三部分，作者引入了turtle模块，这是一个用于图形编程的工具。通过创建小海龟对象，学生可以学习基础的图形绘制，如绘制第一只海龟的程序，以及如何创建和管理多个海龟实例。书中进一步解释了for循环在控制海龟移动和绘制形状中的应用，从而让学生实践编程逻辑和控制流程。《Python Programming》是一本全面的教材，涵盖了Python编程的各个方面，适合初学者和有一定经验的学生作为学习资料，通过实际操作和练习，帮助他们建立起坚实的编程基础。书中的练习题和词汇表也有助于巩固学习内容，促进理论与实践的结合。

FOREWORD

By David Beazley

As an educator, researcher, and book author, I am delighted to see the completion of this book.

Python is a fun and extremely easy-to-use programming language that has steadily gained

in popularity over the last few years. Developed over ten years ago by Guido van Rossum,

Python’s simple syntax and overall feel is largely derived from ABC, a teaching language that

was developed in the 1980’s. However, Python was also created to solve real problems and it

borrows a wide variety of features from programming languages such as C++, Java, Modula-3,

and Scheme. Because of this, one of Python’s most remarkable features is its broad appeal to

professional software developers, scientists, researchers, artists, and educators.

Despite Python’s appeal to many different communities, you may still wonder why Python? or

why teach programming with Python? Answering these questions is no simple task—especially

when popular opinion is on the side of more masochistic alternatives such as C++ and Java.

However, I think the most direct answer is that programming in Python is simply a lot of fun

and more productive.

When I teach computer science courses, I want to cover important concepts in addition to mak-

ing the material interesting and engaging to students. Unfortunately, there is a tendency for

introductory programming courses to focus far too much attention on mathematical abstraction

and for students to become frustrated with annoying problems related to low-level details of

syntax, compilation, and the enforcement of seemingly arcane rules. Although such abstrac-

tion and formalism is important to professional software engineers and students who plan to

continue their study of computer science, taking such an approach in an introductory course

mostly succeeds in making computer science boring. When I teach a course, I don’t want to

have a room of uninspired students. I would much rather see them trying to solve interesting

problems by exploring different ideas, taking unconventional approaches, breaking the rules,

and learning from their mistakes. In doing so, I don’t want to waste half of the semester try-

ing to sort out obscure syntax problems, unintelligible compiler error messages, or the several

hundred ways that a program might generate a general protection fault.

One of the reasons why I like Python is that it provides a really nice balance between the prac-

tical and the conceptual. Since Python is interpreted, beginners can pick up the language and

start doing neat things almost immediately without getting lost in the problems of compila-

tion and linking. Furthermore, Python comes with a large library of modules that can be used

to do all sorts of tasks ranging from web-programming to graphics. Having such a practical

focus is a great way to engage students and it allows them to complete signiﬁcant projects.

iii

PREFACE

By Jeffrey Elkner

This book owes its existence to the collaboration made possible by the Internet and the free

software movement. Its three authors—a college professor, a high school teacher, and a profes-

sional programmer—never met face to face to work on it, but we have been able to collaborate

closely, aided by many other folks who have taken the time and energy to send us their feed-

back.

We think this book is a testament to the beneﬁts and future possibilities of this kind of col-

laboration, the framework for which has been put in place by Richard Stallman and the Free

Software Foundation.

0.0.1 How and why I came to use Python

In 1999, the College Board’s Advanced Placement (AP) Computer Science exam was given in

C++ for the ﬁrst time. As in many high schools throughout the country, the decision to change

languages had a direct impact on the computer science curriculum at Yorktown High School in

Arlington, Virginia, where I teach. Up to this point, Pascal was the language of instruction in

both our ﬁrst-year and AP courses. In keeping with past practice of giving students two years of

exposure to the same language, we made the decision to switch to C++ in the ﬁrst year course

for the 1997-98 school year so that we would be in step with the College Board’s change for

the AP course the following year.

Two years later, I was convinced that C++ was a poor choice to use for introducing students

to computer science. While it is certainly a very powerful programming language, it is also an

extremely difﬁcult language to learn and teach. I found myself constantly ﬁghting with C++’s

difﬁcult syntax and multiple ways of doing things, and I was losing many students unneces-

sarily as a result. Convinced there had to be a better language choice for our ﬁrst-year class, I

went looking for an alternative to C++.

I needed a language that would run on the machines in our GNU/Linux lab as well as on the

Windows and Macintosh platforms most students have at home. I wanted it to be free software,

so that students could use it at home regardless of their income. I wanted a language that was

used by professional programmers, and one that had an active developer community around it.

It had to support both procedural and object-oriented programming. And most importantly, it

How to Think Like a Computer Scientist: Learning with Python 3

Documentation, Release 3rd Edition

had to be easy to learn and teach. When I investigated the choices with these goals in mind,

Python stood out as the best candidate for the job.

I asked one of Yorktown’s talented students, Matt Ahrens, to give Python a try. In two months

he not only learned the language but wrote an application called pyTicket that enabled our

staff to report technology problems via the Web. I knew that Matt could not have ﬁnished an

application of that scale in so short a time in C++, and this accomplishment, combined with

Matt’s positive assessment of Python, suggested that Python was the solution I was looking for.

0.0.2 Finding a textbook

Having decided to use Python in both of my introductory computer science classes the follow-

ing year, the most pressing problem was the lack of an available textbook.

Free documents came to the rescue. Earlier in the year, Richard Stallman had introduced me

to Allen Downey. Both of us had written to Richard expressing an interest in developing free

educational materials. Allen had already written a ﬁrst-year computer science textbook, How

to Think Like a Computer Scientist. When I read this book, I knew immediately that I wanted

to use it in my class. It was the clearest and most helpful computer science text I had seen.

It emphasized the processes of thought involved in programming rather than the features of a

particular language. Reading it immediately made me a better teacher.

How to Think Like a Computer Scientist was not just an excellent book, but it had been released

under the GNU public license, which meant it could be used freely and modiﬁed to meet the

needs of its user. Once I decided to use Python, it occurred to me that I could translate Allen’s

original Java version of the book into the new language. While I would not have been able

to write a textbook on my own, having Allen’s book to work from made it possible for me to

do so, at the same time demonstrating that the cooperative development model used so well in

software could also work for educational materials.

Working on this book for the last two years has been rewarding for both my students and me,

and my students played a big part in the process. Since I could make instant changes whenever

someone found a spelling error or difﬁcult passage, I encouraged them to look for mistakes

in the book by giving them a bonus point each time they made a suggestion that resulted in

a change in the text. This had the double beneﬁt of encouraging them to read the text more

carefully and of getting the text thoroughly reviewed by its most important critics, students

using it to learn computer science.

For the second half of the book on object-oriented programming, I knew that someone with

more real programming experience than I had would be needed to do it right. The book sat

in an unﬁnished state for the better part of a year until the open source community once again

provided the needed means for its completion.

I received an email from Chris Meyers expressing interest in the book. Chris is a professional

programmer who started teaching a programming course last year using Python at Lane Com-

munity College in Eugene, Oregon. The prospect of teaching the course had led Chris to the

book, and he started helping out with it immediately. By the end of the school year he had

created a companion project on our Website at http://openbookproject.net called *Python for

Fun* and was working with some of my most advanced students as a master teacher, guiding

them beyond where I could take them.

vi CONTENTS

How to Think Like a Computer Scientist: Learning with Python 3

Documentation, Release 3rd Edition

0.0.3 Introducing programming with Python

The process of translating and using How to Think Like a Computer Scientist for the past

two years has conﬁrmed Python’s suitability for teaching beginning students. Python greatly

simpliﬁes programming examples and makes important programming ideas easier to teach.

The ﬁrst example from the text illustrates this point. It is the traditional hello, world program,

which in the Java version of the book looks like this:

class Hello {

public static void main (String[] args) {

System.out.println ("Hello, world.");

}

in the Python version it becomes:

print("Hello, World!")

Even though this is a trivial example, the advantages of Python stand out. Yorktown’s Computer

Science I course has no prerequisites, so many of the students seeing this example are looking at

their ﬁrst program. Some of them are undoubtedly a little nervous, having heard that computer

programming is difﬁcult to learn. The Java version has always forced me to choose between

two unsatisfying options: either to explain the class Hello, public static void main, String[]

args, {, and }, statements and risk confusing or intimidating some of the students right at the

start, or to tell them, Just don’t worry about all of that stuff now; we will talk about it later,

and risk the same thing. The educational objectives at this point in the course are to introduce

students to the idea of a programming statement and to get them to write their ﬁrst program,

thereby introducing them to the programming environment. The Python program has exactly

what is needed to do these things, and nothing more.

Comparing the explanatory text of the program in each version of the book further illustrates

what this means to the beginning student. There are seven paragraphs of explanation of Hello,

world! in the Java version; in the Python version, there are only a few sentences. More im-

portantly, the missing six paragraphs do not deal with the big ideas in computer programming

but with the minutia of Java syntax. I found this same thing happening throughout the book.

Whole paragraphs simply disappear from the Python version of the text because Python’s much

clearer syntax renders them unnecessary.

Using a very high-level language like Python allows a teacher to postpone talking about low-

level details of the machine until students have the background that they need to better make

sense of the details. It thus creates the ability to put ﬁrst things ﬁrst pedagogically. One of

the best examples of this is the way in which Python handles variables. In Java a variable is

a name for a place that holds a value if it is a built-in type, and a reference to an object if

it is not. Explaining this distinction requires a discussion of how the computer stores data.

Thus, the idea of a variable is bound up with the hardware of the machine. The powerful and

fundamental concept of a variable is already difﬁcult enough for beginning students (in both

computer science and algebra). Bytes and addresses do not help the matter. In Python a variable

is a name that refers to a thing. This is a far more intuitive concept for beginning students and

is much closer to the meaning of variable that they learned in their math courses. I had much

CONTENTS vii

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