Python编程入门指南（Guido van Rossum）

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"Python简介（Guido van Rossum）An Introduction to Python (Guido van Rossum) 是一本针对初学者的Python编程指南，由Python的创造者Guido van Rossum和Python开发团队编写。该教程涵盖了Python 3.7.0版本的核心概念，包括其易学性、面向对象特性、强大的功能以及简洁的语法。教程强调了Python中的模块、类、异常处理、高级数据类型和动态类型系统。" Python是一种广泛使用的高级编程语言，以其简洁和高效的语法而受到青睐。它支持面向对象编程，允许创建模块和类来组织代码，便于复用和维护。Python的动态类型系统意味着变量的类型可以在运行时改变，增加了编程的灵活性。本教程分为多个章节，逐步引导读者了解Python的基础知识： 1. Whetting Your Appetite：这部分激起读者对Python的兴趣，概述了Python的特点和用途。 2. Using the Python Interpreter：解释如何启动和使用Python解释器，以及它的工作环境。 3. An Informal Introduction to Python：介绍Python的基本用法，如作为计算器进行简单运算，以及初步编程概念。 - 使用Python进行计算：展示了Python的算术运算符和表达式。 - 第一步编程：讲解变量、赋值、输入/输出和基本控制结构。 4. More Control Flow Tools：深入讲解流程控制，包括条件语句（if）、循环（for和while）、range函数、break和continue语句、pass语句以及函数定义。 5. Data Structures：详细讨论Python的数据结构，如列表、元组、序列、集合和字典。 - 列表：可变的有序集合，支持索引和切片操作。 - del语句：用于删除变量或列表元素。 - 元组和序列：不可变的有序集合，常用于数据打包和解包。 - 集合：无序且不重复的元素集。 - 字典：键值对的映射，提供高效查找。 - 循环技巧：涵盖不同的遍历方式，如for-in循环、while循环以及列表推导式。 6. Modules：介绍如何使用和创建模块，以及标准库的使用。 - 更多模块知识：模块是代码组织的基本单元，可以导入和导出。 - 标准模块：Python自带的一系列有用模块，如os、sys、math等。 - dir()函数：用于查看模块或对象的属性和方法。 - 包：用于组织模块的层次结构，提高代码管理。 7. Input and Output：讲解Python中的输入输出操作，包括基本的文件操作和格式化输出。本教程还涉及了更高级的主题，如错误和异常处理、函数参数、类和对象，以及更复杂的编程技术。对于想要入门Python编程的初学者，这是一份宝贵的资源，提供了全面而详尽的指导。通过学习，读者不仅可以掌握Python基础，还能了解到良好的编程风格和实践，为进阶学习打下坚实基础。

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Python Tutorial, Release 3.7.0

Division (/) always returns a ﬂoat. To do ﬂoor division and get an integer result (discarding any fractional

result) you can use the // operator; to calculate the remainder you can use %:

>>> 17 / 3 # classic division returns a float

5.666666666666667

>>>

>>> 17 // 3 # floor division discards the fractional part

>>> 17 % 3 # the % operator returns the remainder of the division

>>> 5 * 3 + 2 # result * divisor + remainder

With Python, it is possible to use the ** operator to calculate powers

>>> 5 ** 2 # 5 squared

>>> 2 ** 7 # 2 to the power of 7

128

The equal sign (=) is used to assign a value to a variable. Afterwards, no result is displayed before the next

interactive prompt:

>>> width = 20

>>> height = 5 * 9

>>> width * height

900

If a variable is not “deﬁned” (assigned a value), trying to use it will give you an error:

>>> n # try to access an undefined variable

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

NameError: name 'n' is not defined

There is full support for ﬂoating point; operators with mixed type operands convert the integer operand to

ﬂoating point:

>>> 4 * 3.75 - 1

14.0

In interactive mode, the last printed expression is assigned to the variable _. This means that when you are

using Python as a desk calculator, it is somewhat easier to continue calculations, for example:

>>> tax = 12.5 / 100

>>> price = 100.50

>>> price * tax

12.5625

>>> price + _

113.0625

>>> round(_, 2)

113.06

This variable should be treated as read-only by the user. Don’t explicitly assign a value to it — you

would create an independent local variable with the same name masking the built-in variable with its magic

behavior.

Since ** has higher precedence than -, -3**2 will be interpreted as -(3**2) and thus result in -9. To avoid this and get

9, you can use (-3)**2.

10 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 3.7.0

In addition to int and float, Python supports other types of numbers, such as Decimal and Fraction.

Python also has built-in support for complex numbers, and uses the j or J suﬃx to indicate the imaginary

part (e.g. 3+5j).

3.1.2 Strings

Besides numbers, Python can also manipulate strings, which can be expressed in several ways. They can be

enclosed in single quotes ('...') or double quotes ("...") with the same result

. \ can be used to escape

quotes:

>>> 'spam eggs' # single quotes

'spam eggs'

>>> 'doesn\'t' # use \' to escape the single quote...

"doesn't"

>>> "doesn't" # ...or use double quotes instead

"doesn't"

>>> '"Yes," they said.'

'"Yes," they said.'

>>> "\"Yes,\" they said."

'"Yes," they said.'

>>> '"Isn\'t," they said.'

'"Isn\'t," they said.'

In the interactive interpreter, the output string is enclosed in quotes and special characters are escaped with

backslashes. While this might sometimes look diﬀerent from the input (the enclosing quotes could change),

the two strings are equivalent. The string is enclosed in double quotes if the string contains a single quote

and no double quotes, otherwise it is enclosed in single quotes. The print() function produces a more

readable output, by omitting the enclosing quotes and by printing escaped and special characters:

>>> '"Isn\'t," they said.'

'"Isn\'t," they said.'

>>> print('"Isn\'t," they said.')

"Isn't," they said.

>>> s = 'First line.\nSecond line.' # \n means newline

>>> s # without print(), \n is included in the output

'First line.\nSecond line.'

>>> print(s) # with print(), \n produces a new line

First line.

Second line.

If you don’t want characters prefaced by \ to be interpreted as special characters, you can use raw strings

by adding an r before the ﬁrst quote:

>>> print('C:\some\name') # here \n means newline!

C:\some

ame

>>> print(r'C:\some\name') # note the r before the quote

C:\some\name

String literals can span multiple lines. One way is using triple-quotes: """...""" or '''...'''. End of

lines are automatically included in the string, but it’s possible to prevent this by adding a \ at the end of

the line. The following example:

Unlike other languages, special characters such as \n have the same meaning with both single ('...') and double ("...")

quotes. The only diﬀerence between the two is that within single quotes you don’t need to escape " (but you have to escape

\') and vice versa.

3.1. Using Python as a Calculator 11

Python Tutorial, Release 3.7.0

>>> word[-1] # last character

'n'

>>> word[-2] # second-last character

'o'

>>> word[-6]

'P'

Note that since -0 is the same as 0, negative indices start from -1.

In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters,

slicing allows you to obtain substring:

>>> word[0:2] # characters from position 0 (included) to 2 (excluded)

'Py'

>>> word[2:5] # characters from position 2 (included) to 5 (excluded)

'tho'

Note how the start is always included, and the end always excluded. This makes sure that s[:i] + s[i:]

is always equal to s:

>>> word[:2] + word[2:]

'Python'

>>> word[:4] + word[4:]

'Python'

Slice indices have useful defaults; an omitted ﬁrst index defaults to zero, an omitted second index defaults

to the size of the string being sliced.

>>> word[:2] # character from the beginning to position 2 (excluded)

'Py'

>>> word[4:] # characters from position 4 (included) to the end

'on'

>>> word[-2:] # characters from the second-last (included) to the end

'on'

One way to remember how slices work is to think of the indices as pointing between characters, with the left

edge of the ﬁrst character numbered 0. Then the right edge of the last character of a string of n characters

has index n, for example:

+---+---+---+---+---+---+

| P | y | t | h | o | n |

+---+---+---+---+---+---+

0 1 2 3 4 5 6

-6 -5 -4 -3 -2 -1

The ﬁrst row of numbers gives the position of the indices 0…6 in the string; the second row gives the

corresponding negative indices. The slice from i to j consists of all characters between the edges labeled i

and j, respectively.

For non-negative indices, the length of a slice is the diﬀerence of the indices, if both are within bounds. For

example, the length of word[1:3] is 2.

Attempting to use an index that is too large will result in an error:

>>> word[42] # the word only has 6 characters

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

IndexError: string index out of range

3.1. Using Python as a Calculator 13

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Python编程入门指南（Guido van Rossum）

[Python入门指南].(Python.Tutorial).Guido.Van.Rossum.

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An Introduction to Python

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