Python入门指南：24小时掌握基础与实践

Python

Tutorial

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本资源是一份针对Python初学者的入门教程，名为"Python Tutorial"，由Guido van Rossum编写，由Fred L. Drake Jr.编辑，于2011年4月18日发布，由Python Software Foundation提供。这份教程旨在帮助读者在24小时内掌握Python的基础知识，迅速进入这个编程语言的神奇世界。 1. **入门引导**：教程首先激发读者的兴趣，通过实际操作和示例介绍Python的基本概念，让新手感受到其易学易用的特点。 2. **Python解释器使用**：章节2重点讲解如何使用Python解释器。包括如何启动解释器（` Invoking the Interpreter`）以及理解解释器环境（`The Interpreter and Its Environment`），这是学习编程的第一步。 3. **Python基础应用**：在3.1节中，教程展示了如何像计算器一样使用Python进行基本数学运算，这有助于初学者建立对Python作为通用编程语言的理解。 4. **控制流工具**：在第4章，作者深入讲解了各种控制流程工具，如`if`语句、`for`循环、`range()`函数、`break`和`continue`等，以及`pass`语句和函数定义。同时，还强调了编码风格的重要性。 5. **数据结构**：这部分是核心内容，涵盖了列表（及其深入）、`del`语句、元组和序列的概念、集合、字典、循环技巧、条件判断的进一步扩展，以及比较序列和数据类型的方法。 6. **模块与导入**：在第6章，教程介绍了模块的使用，包括标准库模块的探索、`dir()`函数的作用以及Python包的导入和管理，这些都是组织代码和重用代码的关键。 7. **输入与输出**：最后一部分探讨了如何处理用户输入和输出，包括更高级的输出格式化技术，使读者能够创建交互式程序并理解数据的输入和处理过程。这份Python入门教程详细而全面地覆盖了从基础语法到数据结构、控制流、模块系统以及基本的I/O操作，适合初学者循序渐进地学习Python编程，并逐步掌握其核心概念和实践技巧。

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Python Tutorial, Release 2.7.1

>>> 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.

>>> 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 2.7.1

>>> 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 2.7.1

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>’

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.

12 Chapter 3. An Informal Introduction to Python

Python Tutorial, Release 2.7.1

>>> word[-0] # (since -0 equals 0)

’H’

Out-of-range negative slice indices are truncated, but don’t try this for single-element (non-slice) indices:

>>> word[-100:]

’HelpA’

>>> word[-10] # error

Traceback (most recent call last):

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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