Python3.4教程：入门到进阶指南

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"Python3.4 Tutorial指导手册英文版，由Guido van Rossum编写，Fred L. Drake, Jr.编辑，2014年7月11日发布，是Python Software Foundation的一部分。该手册涵盖了Python 3.4的基础知识，包括解释器的使用、编程入门、控制流工具、数据结构、模块以及输入和输出等内容。" Python 3.4教程是一个详尽的指南，旨在帮助初学者和经验丰富的程序员了解和掌握Python 3.4版本的编程语言。本教程由Python的创造者Guido van Rossum撰写，由Fred L. Drake, Jr.编辑，确保了内容的专业性和权威性。 **2. 使用Python解释器** 这部分介绍了如何启动Python解释器以及它的工作环境。Python解释器可以作为交互式 shell 使用，允许用户即时测试代码片段，也可以用来执行保存在文件中的脚本。解释器的使用方式包括直接命令行启动和通过脚本执行。 **3. 非正式的Python介绍** 这一章深入浅出地介绍了Python的基本概念，首先展示了Python作为计算器的功能，演示了简单的算术运算。然后逐步引导读者进行更复杂的编程，如变量、条件语句、循环等。 **4. 更多控制流工具** 这一部分详细讲解了如何使用`if`语句进行条件判断，`for`循环遍历序列，`range()`函数生成数字序列，以及`break`、`continue`和`else`子句在循环中的作用。此外，还介绍了`pass`语句，用于创建占位符或空操作。 **5. 数据结构** 数据结构是Python的强大之处，包括列表、元组、序列、集合和字典。列表是最常用的可变序列，可以容纳不同类型的元素；元组是不可变序列，常用于返回多个值或作为键；`del`语句用于删除列表中的元素或整个对象；集合是无序且不重复的元素集；字典则是一系列键值对，提供快速查找功能。 **6. 模块** 模块是Python代码组织的重要方式，可以将相关功能封装在一起。本章讲述了如何导入和使用已有的模块，以及如何创建和使用标准模块。`dir()`函数用于查看模块中的定义，而包则是模块的高级组织形式，允许层次化的命名空间。 **7. 输入与输出** 输入和输出是程序与外界交互的关键。本章涉及格式化输出，如字符串格式化技巧，以及文件操作，包括打开、读取、写入和关闭文件。还讨论了错误处理和异常处理，以确保程序的健壮性。这个Python 3.4教程不仅适合初学者，也对有经验的开发者有价值，因为它覆盖了Python语言的基础和核心特性。通过学习，读者将能够熟练地运用Python进行各种软件开发任务。

Python Tutorial, Release 3.4.1

>>>

>>> 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

>>> 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:

>>> 3

3.75 / 1.5

7.5

>>> 7.0 / 2

3.5

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.

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).

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)

10 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 3.4.1

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," he said.’

’"Yes," he said.’

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

’"Yes," he said.’

>>> ’"Isn\’t," she said.’

’"Isn\’t," she said.’

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

slashes. 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," she said.’

’"Isn\’t," she said.’

>>> print(’"Isn\’t," she said.’)

"Isn’t," she 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:

print("""\

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

""")

produces the following output (note that the initial newline is not included):

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

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 3.4.1

>>> 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 a index that is too large will result in an error:

>>> word[42] # the word only has 7 characters

Traceback (most recent call last):

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

IndexError: string index out of range

However, out of range slice indexes are handled gracefully when used for slicing:

>>> word[4:42]

’on’

>>> word[42:]

’’

Python strings cannot be changed — they are immutable. Therefore, assigning to an indexed position in the string

results in an error:

>>> word[0] = ’J’

...

TypeError: ’str’ object does not support item assignment

>>> word[2:] = ’py’

...

TypeError: ’str’ object does not support item assignment

If you need a different string, you should create a new one:

>>> ’J’ + word[1:]

’Jython’

>>> word[:2] + ’py’

’Pypy’

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

>>> s = ’supercalifragilisticexpialidocious’

>>> len(s)

See also:

3.1. Using Python as a Calculator 13

Python Tutorial, Release 3.4.1

textseq Strings are examples of sequence types, and support the common operations supported by such types.

string-methods Strings support a large number of methods for basic transformations and searching.

string-formatting Information about string formatting with str.format() is described here.

old-string-formatting The old formatting operations invoked when strings and Unicode strings are the left

operand of the % operator are described in more detail here.

3.1.3 Lists

Python knows a number of compound data types, used to group together other values. The most versatile is the

list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain

items of different types, but usually the items all have the same type.

>>> squares = [1, 4, 9, 16, 25]

>>> squares

[1, 4, 9, 16, 25]

Like strings (and all other built-in sequence type), lists can be indexed and sliced:

>>> squares[0] # indexing returns the item

>>> squares[-1]

>>> squares[-3:] # slicing returns a new list

[9, 16, 25]

All slice operations return a new list containing the requested elements. This means that the following slice returns

a new (shallow) copy of the list:

>>> squares[:]

[1, 4, 9, 16, 25]

Lists also supports operations like concatenation:

>>> squares + [36, 49, 64, 81, 100]

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Unlike strings, which are immutable, lists are a mutable type, i.e. it is possible to change their content:

>>> cubes = [1, 8, 27, 65, 125] # something’s wrong here

>>> 4

3 # the cube of 4 is 64, not 65!

>>> cubes[3] = 64 # replace the wrong value

>>> cubes

[1, 8, 27, 64, 125]

You can also add new items at the end of the list, by using the append() method (we will see more about

methods later):

>>> cubes.append(216) # add the cube of 6

>>> cubes.append(7

3) # and the cube of 7

>>> cubes

[1, 8, 27, 64, 125, 216, 343]

Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:

>>> letters = [’a’, ’b’, ’c’, ’d’, ’e’, ’f’, ’g’]

>>> letters

[’a’, ’b’, ’c’, ’d’, ’e’, ’f’, ’g’]

>>> # replace some values

>>> letters[2:5] = [’C’, ’D’, ’E’]

>>> letters

[’a’, ’b’, ’C’, ’D’, ’E’, ’f’, ’g’]

14 Chapter 3. An Informal Introduction to Python

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