Python编程设计指南：思考计算机科学家的方式（2009）

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"Python for Software Design - How to Think Like a Computer Scientist (2009) 是一本由 Allen B. Downey 所著的教程，专为软件设计师和计算机科学家量身打造，强调如何运用Python语言进行高效且逻辑清晰的编程。该书旨在教授读者如何像计算机科学家一样思考问题，并通过实例和理论相结合的方式，深入理解编程基础概念。书中的内容分为多个章节，涵盖了从编程入门到高级技巧的全面指导。第1章介绍了Python编程语言的基本概念，包括程序的本质、调试方法以及自然语言与形式语言的区别。接着，读者会学习变量、表达式和语句，理解数据类型、变量命名规则、运算符及其应用，以及如何进行有效的表达式计算。后续章节深入探讨函数的概念，包括函数调用、参数传递、返回值和类型转换等，以及如何设计模块化和可复用的代码。书中还以实际案例“TurtleWorld”为例，讲解接口设计原则，如封装、泛化和抽象，以及如何通过重构来优化代码结构。条件语句和递归在第5章中讨论，涉及布尔表达式、逻辑运算符、循环控制结构，以及如何使用递归来解决复杂问题。此外，书中强调了函数的回报值、函数设计的层次性和正确处理递归过程的重要性。迭代、字符串处理、列表、元组、字典等数据结构在后续章节逐一介绍，通过实例展示如何操作这些数据结构，以及它们在解决问题时的应用。书中穿插了多个案例研究，如“WordPlay”和“DataStructureSelection”，帮助读者将理论知识应用到实际项目中，提高软件设计能力。书的末尾部分介绍了文件处理，包括数据持久化、输入输出操作、异常处理以及与数据库的交互。此外，还提到了序列化（pickle）和进程间通信（pipes）等高级主题。这本书不仅是一本Python编程指南，更是一部引导读者培养逻辑思维和软件设计思维的宝典，适合初学者和经验丰富的开发者阅读和提升。"

The Way of the Program

The goal of this book is to teach you to think like a computer scientist. This way

of thinking combines some of the best features of mathematics, engineering, and

natural science. Like mathematicians, computer scientists use formal languages to

denote ideas (speciﬁcally computations). Like engineers, they design things, assem-

bling components into systems and evaluating tradeoffs among alternatives. Like

scientists, they observe the behavior of complex systems, form hypotheses, and test

predictions.

The single most important skill for a computer scientist is problem solving. Problem

solving means the ability to formulate problems, think creatively about solutions,

and express a solution clearly and accurately. As it turns out, the process of learning

to program is an excellent opportunity to practice problem-solving skills. That’s why

this chapter is called “The way of the program.”

On one level, you will be learning to program, a useful skill by itself. On another

level, you will use programming as a means to an end. As we go along, that end will

become clearer.

1.1 THE PYTHON PROGRAMMING LANGUAGE

The programming language you will learn is Python. Python is an example of a high-

level language; other high-level languages you might have heard of are C, C++, Perl,

and Java.

There are also low-level languages, sometimes referred to as “machine languages”

or “assembly languages.” Loosely speaking, computers can only execute programs

written in low-level languages. So programs written in a high-level language have to

be processed before they can run. This extra processing takes some time, which is a

small disadvantage of high-level languages.

The advantages are enormous, however. First, it is much easier to program in a high-

level language. Programs written in a high-level language take less time to write,

2 The Way of the Program

they are shorter and easier to read, and they are more likely to be correct. Second,

high-level languages are portable, meaning that they can run on different kinds of

computers with few or no modiﬁcations. Low-level programs can run on only one

kind of computer and have to be rewritten to run on another.

Due to these advantages, almost all programs are written in high-level languages.

Low-level languages are used only for a few specialized applications.

Two kinds of programs process high-level languages into low-level languages: inter-

preters and compilers. An interpreter reads a high-level program and executes it,

meaning that it does what the program says. It processes the program a little at a

time, alternately reading lines and performing computations.

OUTPUT

SOURCE

CODE

INTERPRETER

A compiler reads the program and translates it completely before the program starts

running. In this context, the high-level program is called the source code, and the

translated program is called the object code or the executable. Once a program is

compiled, you can execute it repeatedly without further translation.

OUTPUT

CODE

OBJECT

EXECUTOR

CODE

SOURCE

COMPILER

Python is considered an interpreted language because Python programs are executed

by an interpreter. There are two ways to use the interpreter: interactive mode and

script mode. In interactive mode, you type Python programs and the interpreter

prints the result:

>>>1+1

The chevron, >>>, is the prompt the interpreter uses to indicate that it is ready. If

you type 1+1, the interpreter replies 2.

Alternatively, you can store code in a ﬁle and use the interpreter to execute the

contents of the ﬁle, which is called a script. By convention, Python scripts have

names that end with .py.

To execute the script, you have to tell the interpreter the name of the ﬁle. In a UNIX

command window, you would type python dinsdale.py. In other development

environments, the details of executing scripts are different. You can ﬁnd instructions

for your environment at the Python Website python.org.

1.3 What is Debugging 3

Working in interactive mode is convenient for testing small pieces of code because

you can type and execute them immediately. But for anything more than a few lines,

you should save your code as a script so you can modify and execute it in the future.

1.2 WHAT IS A PROGRAM?

A program is a sequence of instructions that speciﬁes how to perform a computa-

tion. The computation might be something mathematical, such as solving a system of

equations or ﬁnding the roots of a polynomial, but it can also be a symbolic compu-

tation, such as searching for and replacing text in a document or (strangely enough)

compiling a program.

The details look different in different languages, but a few basic instructions appear

in just about every language:

input: Get data from the keyboard, a ﬁle, or some other device.

output: Display data on the screen or send data to a ﬁle or other device.

math: Perform basic mathematical operations like addition and multiplication.

conditional execution: Check for certain conditions and execute the appropriate

sequence of statements.

repetition: Perform some action repeatedly, usually with some variation.

Believe it or not, that’s pretty much all there is to it. Every program you’ve ever used,

no matter how complicated, is made up of instructions that look pretty much like

these. So you can think of programming as the process of breaking a large, complex

task into smaller and smaller subtasks until the subtasks are simple enough to be

performed with one of these basic instructions.

That may be a little vague, but we will come back to this topic when we talk about

algorithms.

1.3 WHAT IS DEBUGGING?

Programming is error-prone. For whimsical reasons, programming errors are called

bugs and the process of tracking them down is called debugging.

Three kinds of errors can occur in a program: syntax errors, runtime errors, and

semantic errors. It is useful to distinguish between them in order to track them down

more quickly.

1.3.1 Syntax Errors

Python can only execute a program if the syntax is correct; otherwise, the interpreter

displays an error message. Syntax refers to the structure of a program and the rules

about that structure. For example, parentheses have to come in matching pairs, so

(1+2)is legal, but 8) is a syntax error.

In English, readers can tolerate most syntax errors, which is why we can read the

poetry of e. e. cummings without spewing error messages. Python is not so forgiving.

4 The Way of the Program

If there is a single syntax error anywhere in your program, Python will display an

error message and quit, and you will not be able to run your program. During the

ﬁrst few weeks of your programming career, you will probably spend a lot of time

tracking down syntax errors. As you gain experience, you will make fewer errors and

ﬁnd them faster.

1.3.2 Runtime Errors

The second type of error is a runtime error, so called because the error does not

appear until after the program has started running. These errors are also called

exceptions because they usually indicate that something exceptional (and bad) has

happened.

Runtime errors are rare in the simple programs you will see in the ﬁrst few chapters,

so it might be a while before you encounter one.

1.3.3 Semantic Errors

The third type of error is the semantic error. If there is a semantic error in your

program, it will run successfully in the sense that the computer will not generate any

error messages, but it will not do the right thing. It will do something else. Speciﬁcally,

it will do what you told it to do.

The problem is that the program you wrote is not the program you wanted to write.

The meaning of the program (its semantics) is wrong. Identifying semantic errors

can be tricky because it requires you to work backward by looking at the output of

the program and trying to ﬁgure out what it is doing.

1.3.4 Experimental Debugging

One of the most important skills you will acquire is debugging. Although it can

be frustrating, debugging is one of the most intellectually rich, challenging, and

interesting parts of programming.

In some ways, debugging is like detective work. You are confronted with clues, and

you have to infer the processes and events that led to the results you see.

Debugging is also like an experimental science. Once you have an idea about what is

going wrong, you modify your program and try again. If your hypothesis was correct,

then you can predict the result of the modiﬁcation, and you take a step closer to a

working program. If your hypothesis was wrong, you have to come up with a new

one. As Sherlock Holmes pointed out, “When you have eliminated the impossible,

whatever remains, however improbable, must be the truth” (A. Conan Doyle, The

Sign of Four).

For some people, programming and debugging are the same thing. That is, pro-

gramming is the process of gradually debugging a program until it does what you

want. The idea is that you should start with a program that does something and make

1.4 Formal and Natural Languages 5

small modiﬁcations, debugging them as you go, so that you always have a working

program.

For example, Linux is an operating system that contains thousands of lines of code,

but it started out as a simple program Linus Torvalds used to explore the Intel 80386

chip. According to Larry Greenﬁeld, “One of Linus’s earlier projects was a program

that would switch between printing AAAA and BBBB. This later evolved to Linux”

(The Linux Users’ Guide Beta Version 1).

Later chapters will make more suggestions about debugging and other programming

practices.

1.4 FORMAL AND NATURAL LANGUAGES

Natural languages are the languages people speak, such as English, Spanish, and

French. They were not designed by people (although people try to impose some

order on them); they evolved naturally.

Formal languages are languages that are designed by people for speciﬁc applica-

tions. For example, the notation that mathematicians use is a formal language that is

particularly good at denoting relationships among numbers and symbols. Chemists

use a formal language to represent the chemical structure of molecules. And most

importantly:

Programming languages are formal languages that have been designed to express

computations.

Formal languages tend to have strict rules about syntax. For example, 3 + 3 = 6

is a syntactically correct mathematical statement, but 3+=3$6 is not. H

Oisa

syntactically correct chemical formula, but

Zz is not.

Syntax rules come in two ﬂavors, pertaining to tokens and structure. Tokens are the

basic elements of the language, such as words, numbers, and chemical elements. One

of the problems with 3+=3$6 is that $ is not a legal token in mathematics (at least

as far as I know). Similarly,

Zz is not legal because there is no element with the

abbreviation Zz.

The second type of syntax error pertains to the structure of a statement, that is, the

way the tokens are arranged. The statement 3+=3$6 is illegal because even though

+ and = are legal tokens, you can’t have one right after the other. Similarly, in a

chemical formula the subscript comes after the element name, not before.

Exercise 1.1

Write a well-structured English sentence with invalid tokens in it. Then write another

sentence with all valid tokens but with invalid structure.

When you read a sentence in English or a statement in a formal language, you have

to ﬁgure out what the structure of the sentence is (although in a natural language

you do this subconsciously). This process is called parsing.

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