Python 2.7.7 官方教程：入门与进阶指南

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"Python官方教程，这是一份英文版本的教程，适合初学者快速入门Python编程。" Python是世界上最受欢迎的编程语言之一，以其简洁、易读的语法和强大的功能而闻名。这份官方教程覆盖了从基础到进阶的各种主题，帮助用户全面了解Python。教程的第1部分“Whetting Your Appetite”旨在激发学习者对Python的兴趣，简要介绍Python的一些基本概念和优势。接下来，第2章“Using the Python Interpreter”讲解如何启动和使用Python解释器，包括交互式模式和脚本执行，以及解释器的工作环境。第3章“An Informal Introduction to Python”是Python编程的起点，首先介绍了Python作为计算器的简单用法，然后逐步深入到更复杂的编程概念，如变量、运算符、控制流语句（如if、for、while）和函数定义。第4章“More Control Flow Tools”详细阐述了Python的控制流工具，包括if语句、for循环、range函数、break和continue语句、pass语句，以及函数的进一步使用，强调了编写清晰、整洁代码的风格。第5章“Data Structures”探讨了Python的数据结构，如列表的更多操作、del语句用于删除对象、元组和序列的特性、集合的使用，以及字典——Python中非常重要的映射类型。此外，还介绍了不同的循环技巧和条件比较方法。第6章“Modules”讲解了如何组织和重用代码，通过模块导入和创建自定义模块，以及标准库的使用。dir()函数用于查看模块的属性，而包则提供了一种管理和组织多个相关模块的方式。第7章“Input and Output”涵盖了数据的输入和输出，包括基本的文件操作、更复杂的输出格式化技术，以及错误处理和异常处理，这些都是编写实用程序时不可或缺的部分。这份Python官方教程为初学者提供了一个详尽的指南，涵盖了Python编程的基础和核心概念，帮助学习者快速上手并深入理解Python语言。无论是想进行数据分析、网络编程还是软件开发，这个教程都是一个理想的起点。

Python Tutorial, Release 2.7.7

>>> 7/-3

-3

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

>>> width

height

900

A value can be assigned to several variables simultaneously:

>>> x = y = z = 0 # Zero x, y and z

>>> x

>>> y

>>> z

Variables must be “deﬁned” (assigned a value) before they can be used, or an error will occur:

>>> 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:

>>> 3

3.75 / 1.5

7.5

>>> 7.0 / 2

3.5

Complex numbers are also supported; imaginary numbers are written with a sufﬁx of j or J. Complex numbers

with a nonzero real component are written as (real+imagj), or can be created with the complex(real,

imag) function.

>>> 1j

(-1+0j)

>>> 1j

complex(0,1)

(-1+0j)

>>> 3+1j

(3+3j)

>>> (3+1j)

(9+3j)

>>> (1+2j)/(1+1j)

(1.5+0.5j)

Complex numbers are always represented as two ﬂoating point numbers, the real and imaginary part. To extract

these parts from a complex number z, use z.real and z.imag.

>>> a=1.5+0.5j

>>> a.real

1.5

>>> a.imag

0.5

The conversion functions to ﬂoating point and integer (float(), int() and long()) don’t work for complex

numbers — there is no one correct way to convert a complex number to a real number. Use abs(z) to get its

magnitude (as a ﬂoat) or z.real to get its real part.

10 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 2.7.7

>>> a=3.0+4.0j

>>> float(a)

Traceback (most recent call last):

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

TypeError: can’t convert complex to float; use abs(z)

>>> a.real

3.0

>>> a.imag

4.0

>>> abs(a) # sqrt(a.real

2 + a.imag

5.0

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.

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:

>>> ’spam eggs’

’spam eggs’

>>> ’doesn\’t’

"doesn’t"

>>> "doesn’t"

"doesn’t"

>>> ’"Yes," he said.’

’"Yes," he said.’

>>> "\"Yes,\" he said."

’"Yes," he said.’

>>> ’"Isn\’t," she said.’

’"Isn\’t," she said.’

The interpreter prints the result of string operations in the same way as they are typed for input: inside quotes, and

with quotes and other funny characters escaped by backslashes, to show the precise value. The string is enclosed

in double quotes if the string contains a single quote and no double quotes, else it’s enclosed in single quotes. The

print statement produces a more readable output for such input strings.

String literals can span multiple lines in several ways. Continuation lines can be used, with a backslash as the last

character on the line indicating that the next line is a logical continuation of the line:

hello = "This is a rather long string containing\n\

several lines of text just as you would do in C.\n\

Note that whitespace at the beginning of the line is\

significant."

print hello

3.1. Using Python as a Calculator 11

Python Tutorial, Release 2.7.7

Note that newlines still need to be embedded in the string using \n – the newline following the trailing backslash

is discarded. This example would print the following:

This is a rather long string containing

several lines of text just as you would do in C.

Note that whitespace at the beginning of the line is significant.

Or, strings can be surrounded in a pair of matching triple-quotes: """ or ”’. End of lines do not need to be

escaped when using triple-quotes, but they will be included in the string.

print """

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

"""

produces the following output:

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

If we make the string literal a “raw” string, \n sequences are not converted to newlines, but the backslash at the

end of the line, and the newline character in the source, are both included in the string as data. Thus, the example:

hello = r"This is a rather long string containing\n\

several lines of text much as you would do in C."

print hello

would print:

This is a rather long string containing\n\

several lines of text much as you would do in C.

Strings can be concatenated (glued together) with the + operator, and repeated with

>>> word = ’Help’ + ’A’

>>> word

’HelpA’

>>> ’<’ + word

5 + ’>’

’<HelpAHelpAHelpAHelpAHelpA>’

Two string literals next to each other are automatically concatenated; the ﬁrst line above could also have been

written word = ’Help’ ’A’; this only works with two literals, not with arbitrary string expressions:

>>> ’str’ ’ing’ # <- This is ok

’string’

>>> ’str’.strip() + ’ing’ # <- This is ok

’string’

>>> ’str’.strip() ’ing’ # <- This is invalid

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

’str’.strip() ’ing’

SyntaxError: invalid syntax

Strings can be subscripted (indexed); like in C, the ﬁrst character of a string has subscript (index) 0. There is no

separate character type; a character is simply a string of size one. Like in Icon, substrings can be speciﬁed with

the slice notation: two indices separated by a colon.

>>> word[4]

’A’

>>> word[0:2]

’He’

>>> word[2:4]

’lp’

12 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 2.7.7

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

IndexError: string index out of range

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:

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

| H | e | l | p | A |

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

0 1 2 3 4 5

-5 -4 -3 -2 -1

The ﬁrst row of numbers gives the position of the indices 0...5 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.

The built-in function len() returns the length of a string:

>>> s = ’supercalifragilisticexpialidocious’

>>> len(s)

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