Python 3学习指南：思维像计算机科学家

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《如何像计算机科学家一样思考：用Python 3学习》是一本由Allen B. Downey、Peter Wentworth、Jeffrey Elkner和Chris Meyers合著的编程指南，针对初学者设计，旨在教授编程思维和Python语言基础。该书于2018年12月发布，是第三版，适合那些希望系统地掌握编程概念并以Python作为入门工具的人。本书共分为16章，详细讲解了程序设计的基础知识： 1. **程序的思维方式**：引导读者理解计算机解决问题的逻辑步骤，强调问题解决的结构化方法。 2. **变量、表达式和语句**：介绍编程中的基本概念，如变量用于存储数据，表达式用于计算结果，语句控制程序流程。 3. **程序流程**：涵盖循环（for和while）和条件语句（if和elif），帮助读者构建具有逻辑分支的程序。 4. **函数**：深入讨论函数的概念，包括定义、参数传递和返回值，强调函数在代码复用和模块化中的作用。 5. **数据类型**：介绍了Python中的基本数据类型，如整数、浮点数、字符串和布尔值，以及列表、元组和字典等复杂数据结构。 6. **NumPy**：讲解科学计算库NumPy，用于处理大型数组和执行高效的数学运算。 7. **文件操作**：涵盖了文件的读写、路径处理等操作，让读者学会与外部数据交互。 8. **模块**：阐述如何导入和使用预定义的模块，以及编写自定义模块来组织代码。 9. **更多数据类型**：扩展至集合、枚举和其他高级数据类型，提升编程灵活性。 10. **递归**：探讨递归算法的概念及其在问题求解中的应用。 11. **类和对象**：介绍面向对象编程（OOP）的概念，包括类、对象、继承和封装。 12. **异常处理**：讲解如何处理程序运行时可能出现的错误，提高代码健壮性。 13. **拟合**：涉及数据拟合和统计学应用，可能涉及Python的统计分析库。 14. **PyGame**：通过游戏开发来学习图形用户界面（GUI）编程和事件驱动编程。 15. **版权和贡献**：版权信息以及如何参与项目贡献的指导。 16. **附录**：包含多个主题的补充材料，如模块、数据类型、递归、类和对象、异常处理、拟合和PyGame的深入讨论。《如何像计算机科学家一样思考：用Python 3学习》不仅教授语言知识，还注重培养读者的逻辑思维、问题解决能力和编程实践技巧，是一本实用且富有启发性的编程入门书籍。

CHAPTER 2

Variables, expressions and statements

2.1 Values and data types

A value is one of the fundamental things — like a letter or a number — that a program manipulates. The values we

have seen so far are 4 (the result when we added 2+2), and "Hello, World!".

These values are classiﬁed into different classes, or data types: 4 is an integer, and "Hello, World!" is a string,

so-called because it contains a string of letters. You (and the interpreter) can identify strings because they are enclosed

in quotation marks.

If you are not sure what class a value falls into, Python has a function called type which can tell you.

>>> type("Hello, World!")

>>> type(17)

Not surprisingly, strings belong to the class str and integers belong to the class int. Less obviously, numbers with a

decimal point belong to a class called ﬂoat, because these numbers are represented in a format called ﬂoating-point.

At this stage, you can treat the words class and type interchangeably. We’ll come back to a deeper understanding of

what a class is in later chapters.

>>> type(3.2)

What about values like "17" and "3.2"? They look like numbers, but they are in quotation marks like strings.

>>> type("17")

>>> type("3.2")

They’re strings!

Strings in Python can be enclosed in either single quotes (') or double quotes ("), or three of each (''' or """)

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

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>>> 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

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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

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(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

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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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