Python 2.7官方教程：从基础到模块与输入输出

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本资源是Python官方文档教程的2.7版本，由Guido van Rossum编写并由Fred L. Drake Jr.编辑，发布于2010年11月27日。Python Software Foundation 提供，邮件联系地址为docs@python.org。这份教程详细介绍了Python语言的基础知识和核心特性。第1章"Whetting Your Appetite"旨在激发读者对Python的兴趣，让初学者了解编程的魅力和实用性。通过简单示例展示如何用Python进行基础计算，使新接触者感受到Python作为脚本语言的直观性和易用性。第2章"Using the Python Interpreter"介绍了如何启动Python解释器以及其环境设置。这部分内容涵盖了命令行输入、交互式模式，以及解释器的基本操作，为后续编程实践打下基础。第3章"An Informal Introduction to Python"深入浅出地介绍了Python作为计算器的使用方法，并引导读者步入编程的第一步，包括基本的算术运算和变量的使用。第4章"More Control Flow Tools"着重讲解了控制流程的关键概念，如if语句、for循环、range函数、break和continue语句，以及pass语句的使用。此外，还讨论了函数的定义和编程风格的重要性。第5章"Data Structures"涵盖了Python中的数据结构，包括列表的深入操作、del语句、元组和序列类型、集合与字典的使用，以及不同数据结构在控制流和条件判断中的应用。这一章强调了数据组织和处理在程序设计中的核心地位。第6章"Modules"讲解了模块的导入和使用，包括标准库模块的介绍，以及dir()函数的作用。此外，还涉及了包的管理和加载，为扩展Python功能和复用代码提供了重要工具。第7章"Input and Output"关注的是如何处理用户输入和输出，通过更复杂的格式化输出，展示了Python在数据交互方面的灵活性。这份教程覆盖了Python语言基础至中级的内容，适合初次接触Python的学员系统学习，同时对于希望进一步提升Python编程技能的开发者也具有参考价值。通过逐步深入的章节，读者可以掌握Python语法、数据结构、控制流和模块管理等方面的知识，从而更好地应用于实际项目中。

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Python Tutorial, Release 2.7.1

>>> # Integer division returns the floor:

... 7/3

>>> 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:

>>> # try to access an undefined variable

... n

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.

10 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 2.7.1

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

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.

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, described later, can be used to write strings without quotes or escapes.)

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

>>> word = ’Help’ + ’A’

>>> word

’HelpA’

>>> ’<’ + word

5 + ’>’

’<HelpAHelpAHelpAHelpAHelpA>’

12 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 2.7.1

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’

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] # The first two characters

’He’

>>> word[2:] # Everything except the first two characters

’lpA’

Unlike a C string, Python strings cannot be changed. Assigning to an indexed position in the string results in an error:

>>> word[0] = ’x’

Traceback (most recent call last):

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

TypeError: object does not support item assignment

>>> word[:1] = ’Splat’

Traceback (most recent call last):

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

TypeError: object does not support slice assignment

However, creating a new string with the combined content is easy and efﬁcient:

>>> ’x’ + word[1:]

’xelpA’

>>> ’Splat’ + word[4]

’SplatA’

Here’s a useful invariant of slice operations: s[:i] + s[i:] equals s.

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

’HelpA’

>>> word[:3] + word[3:]

’HelpA’

Degenerate slice indices are handled gracefully: an index that is too large is replaced by the string size, an upper bound

smaller than the lower bound returns an empty string.

3.1. Using Python as a Calculator 13

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