计算机科学家思维入门：Python 3编程指南

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"《像计算机科学家一样思考——用Python 3学习》是一本面向初学者的Python编程教材，由Peter Wentworth、Jeffrey Elkner、Allen B. Downey和Chris Meyers共同编写。本书涵盖了编程基础到高级主题，包括变量、表达式、语句、程序流程、函数、数据类型、NumPy库、文件操作、模块、更多数据类型、递归、类与对象、异常处理、拟合、PyGame游戏开发等内容。" 本文档是《像计算机科学家一样思考——用Python 3学习》的第三版，旨在帮助读者理解编程思维方式，并通过Python语言实践这些概念。书中详细介绍了以下几个关键知识点： 1. **编程基础**：第1章“程序之道”引导读者理解编程的基本思想，包括问题解决、逻辑推理和算法设计。 2. **变量、表达式和语句**：第2章深入讨论了变量（用于存储数据的容器）、表达式（计算值的组合）以及语句（执行特定任务的代码片段）的概念。 3. **程序流程控制**：第3章介绍了如何控制程序的执行流程，如条件语句（if-else）、循环（for和while）以及分支结构。 4. **函数**：第4章讲解了如何定义和使用函数，这是代码重用和模块化设计的基础。 5. **数据类型**：第5章涵盖了Python中的基本数据类型，如整数、浮点数、字符串、列表、元组、集合和字典。 6. **NumPy**：第6章介绍了一个强大的科学计算库NumPy，它提供了高效的多维数组和矩阵运算。 7. **文件操作**：第7章讲解了如何读写文件，以及如何处理文件错误。 8. **模块**：第8章介绍了Python的模块系统，如何导入和使用其他文件中的代码。 9. **更多数据类型**：在附录B中，进一步探讨了额外的数据类型和相关操作。 10. **递归**：第10章和附录C深入讨论了递归——函数调用自身以解决问题的方法。 11. **类和对象**：第11章介绍了面向对象编程，包括类的定义、对象的创建和继承。 12. **异常处理**：第12章讲述了如何捕获和处理程序运行时的错误，以提高代码的健壮性。 13. **拟合**：第13章涉及数据分析中的数据拟合技术。 14. **PyGame**：第14章介绍了PyGame库，用于创建游戏和图形用户界面。 15. **版权和贡献者列表**：书末包含了版权信息和对贡献者的致谢。这本书是学习Python编程和计算机科学思维的理想资源，不仅覆盖了语言的基本语法，还深入到了更高级的主题，适合初学者和有一定经验的程序员提升技能。通过实际练习和案例，读者可以逐步掌握如何像计算机科学家一样思考和解决问题。

How to Think Like a Computer Scientist: Learning with Python 3 Documentation, Release 3rd

Edition

>>> type('This is a string.')

>>> type("And so is this.")

>>> type("""and this.""")

>>> type('''and even this...''')

Double quoted strings can contain single quotes inside them, as in "Bruce's beard", and single quoted strings

can have double quotes inside them, as in 'The knights who say "Ni!"'.

Strings enclosed with three occurrences of either quote symbol are called triple quoted strings. They can contain either

single or double quotes:

>>> print('''"Oh no", she exclaimed, "Ben's bike is broken!"''')

"Oh no", she exclaimed, "Ben's bike is broken!"

>>>

Triple quoted strings can even span multiple lines:

>>> message = """This message will

... span several

... lines."""

>>> print(message)

This message will

span several

lines.

>>>

Python doesn’t care whether you use single or double quotes or the three-of-a-kind quotes to surround your strings:

once it has parsed the text of your program or command, the way it stores the value is identical in all cases, and the

surrounding quotes are not part of the value. But when the interpreter wants to display a string, it has to decide which

quotes to use to make it look like a string.

>>> 'This is a string.'

'This is a string.'

>>> """And so is this."""

'And so is this.'

So the Python language designers usually chose to surround their strings by single quotes. What do think would

happen if the string already contained single quotes?

When you type a large integer, you might be tempted to use commas between groups of three digits, as in 42,000.

The same goes for entering Dutch-style ﬂoating point numbers using a comma instead of a decimal dot. This is not a

legal integer in Python, but it does mean something else, which is legal:

>>> 42000

42000

>>> 42,000

(42, 0)

Well, that’s not what we expected at all! Because of the comma, Python chose to treat this as a pair of values. We’ll

come back to learn about pairs later. But, for the moment, remember not to put commas or spaces in your integers, no

matter how big they are. Also revisit what we said in the previous chapter: formal languages are strict, the notation is

concise, and even the smallest change might mean something quite different from what you intended.

12 Chapter 2. Variables, expressions and statements

How to Think Like a Computer Scientist: Learning with Python 3 Documentation, Release 3rd

Edition

2.2 Variables

One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a

name that refers to a value.

The assignment statement gives a value to a variable:

>>> message = "What's up, Doc?"

>>> n = 17

>>> pi = 3.14159

This example makes three assignments. The ﬁrst assigns the string value "What's up, Doc?" to a variable

named message. The second gives the integer 17 to n, and the third assigns the ﬂoating-point number 3.14159 to

a variable called pi.

The assignment token, =, should not be confused with equals, which uses the token ==. The assignment statement

binds a name, on the left-hand side of the operator, to a value, on the right-hand side. Basically, an assignment is an

order, and the equals operator can be read as a question mark. This is why you will get an error if you enter:

>>> 17 = n

File "<interactive input>", line 1

SyntaxError: can't assign to literal

Tip: When reading or writing code, say to yourself “n is assigned 17” or “n gets the value 17”. Don’t

say “n equals 17”.

A common way to represent variables on paper is to write the name with an arrow pointing to the variable’s value.

This kind of ﬁgure is called a state snapshot because it shows what state each of the variables is in at a particular

instant in time. (Think of it as the variable’s state of mind). Some editors and programming environments do this for

you, and allow you to click through the state of the program saving you some paper. This diagram shows the result of

executing the assignment statements:

If you ask the interpreter to evaluate a variable, it will produce the value that is currently linked to the variable:

>>> message

'What's up, Doc?'

>>> n

>>> pi

3.14159

We use variables in a program to “remember” things, perhaps the current score at the football game. But variables are

variable. This means they can change over time, just like the scoreboard at a football game. You can assign a value to

a variable, and later assign a different value to the same variable. (This is different from maths. In maths, if you give

‘x‘ the value 3, it cannot change to link to a different value half-way through your calculations!)

>>> day = "Thursday"

>>> day

'Thursday'

(continues on next page)

2.2. Variables 13

How to Think Like a Computer Scientist: Learning with Python 3 Documentation, Release 3rd

Edition

(continued from previous page)

>>> day = "Friday"

>>> day

'Friday'

>>> day = 21

>>> day

You’ll notice we changed the value of day three times, and on the third assignment we even made it refer to a value

that was of a different type.

A great deal of programming is about having the computer remember things, e.g. The number of missed calls on your

phone, and then arranging to update or change the variable when you miss another call.

2.3 Variable names and keywords

Variable names can be arbitrarily long. They can contain both letters and digits, but they have to begin with a letter

or an underscore. Although it is legal to use uppercase letters, by convention we don’t. If you do, remember that case

matters. Bruce and bruce are different variables.

The underscore character ( _) can appear in a name. It is often used in names with multiple words, such as my_name

or price_of_tea_in_china.

There are some situations in which names beginning with an underscore have special meaning, so a safe rule for

beginners is to start all names with a letter.

If you give a variable an illegal name, you get a syntax error:

>>> 76trombones = "big parade"

SyntaxError: invalid syntax

>>> more$ = 1000000

SyntaxError: invalid syntax

>>> class = "Computer Science 101"

SyntaxError: invalid syntax

76trombones is illegal because it does not begin with a letter. more$ is illegal because it contains an illegal

character, the dollar sign. But what’s wrong with class?

It turns out that class is one of the Python keywords. Keywords deﬁne the language’s syntax rules and structure,

and they cannot be used as variable names.

Python has thirty-something keywords (and every now and again improvements to Python introduce or eliminate one

or two):

and as assert break class continue

def del elif else except exec

ﬁnally for from global if import

in is lambda nonlocal not or

pass raise return try while with

yield True False None

You might want to keep this list handy. If the interpreter complains about one of your variable names and you don’t

know why, see if it is on this list.

Programmers generally choose names for their variables that are meaningful to the human readers of the program —

they help the programmer document, or remember, what the variable is used for.

14 Chapter 2. Variables, expressions and statements

How to Think Like a Computer Scientist: Learning with Python 3 Documentation, Release 3rd

Edition

Caution: Beginners sometimes confuse “meaningful to the human readers” with “meaningful to the computer”.

So they’ll wrongly think that because they’ve called some variable average or pi, it will somehow magically

calculate an average, or magically know that the variable pi should have a value like 3.14159. No! The computer

doesn’t understand what you intend the variable to mean.

So you’ll ﬁnd some instructors who deliberately don’t choose meaningful names when they teach beginners —

not because we don’t think it is a good habit, but because we’re trying to reinforce the message that you — the

programmer — must write the program code to calculate the average, and you must write an assignment statement

to give the variable pi the value you want it to have.

e = 3.1415

ray = 10

size = e

ray

pi = 3.1415

radius = 10

area = pi

radius

The above two snippets do exactly the same thing, but the bottom one uses the right kind of variable names. For the

computer there is no difference at all, but for a human, using the names and letters that are part of the conventional

way of writing things make all the difference in the world. Using e instead of pi completely confuses people,

while computers will just perform the calculation!

2.4 Statements

A statement is an instruction that the Python interpreter can execute. We have only seen the assignment statement so

far. Some other kinds of statements that we’ll see shortly are while statements, for statements, if statements, and

import statements. (There are other kinds too!)

When you type a statement on the command line, Python executes it. Statements don’t produce any result.

2.5 Evaluating expressions

An expression is a combination of values, variables, operators, and calls to functions. If you type an expression at the

Python prompt, the interpreter evaluates it and displays the result:

>>> 1 + 1

>>> len("hello")

In this example len is a built-in Python function that returns the number of characters in a string. We’ve previously

seen the print and the type functions, so this is our third example of a function!

The evaluation of an expression produces a value, which is why expressions can appear on the right hand side of

assignment statements. A value all by itself is a simple expression, and so is a variable.

>>> 17

>>> y = 3.14

>>> x = len("hello")

>>> x

(continues on next page)

2.4. Statements 15

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