Python基础-01 - 《Python自动化》

检查Python版本
下载安装 Python
- 安装
认识程序
- 解释器
数字的数学运算
字符串
字符串格式化
- format 格式化
- f 格式化

检查Python版本

在命令行窗口检查 Python版本

python -V
Python 3.8.3

下载安装 Python

官网： https://www.python.org/

https://www.python.org/downloads/release/python-383/

安装

安装成功

认识程序

解释型语言

编译型语言

解释器

数字的数学运算

+	加法	>>> 1+1 2
-	减法	>>> 1-1 0
*	乘法	>>> 2*3 6
/	除法	>>> 10/3 3.3333333333333335
//	整除	>>> 10//3 3
%	取余（取模）	>>> 10%3 1
**	次幂运算	>>> 2**10 1024

数据精度问题

>>> 0.1+0.2
0.30000000000000004

整数 int

整数（比如 2、4、20 ）的类型是 [int](https://docs.python.org/zh-cn/3/library/functions.html#int)，有小数部分的（比如 5.0、1.6 ）的类型是 [float](https://docs.python.org/zh-cn/3/library/functions.html#float)。在这个手册的后半部分我们会看到更多的数字类型。

浮点数 float

除法运算 (/) 永远返回浮点数类型。如果要做 floor division 得到一个整数结果（忽略小数部分）你可以使用 // 运算符；如果要计算余数，可以使用 %

其它数字类型

除了 [int](https://docs.python.org/zh-cn/3/library/functions.html#int) 和 [float](https://docs.python.org/zh-cn/3/library/functions.html#float)，Python也支持其他类型的数字，例如 [Decimal](https://docs.python.org/zh-cn/3/library/decimal.html#decimal.Decimal) 或者 [Fraction](https://docs.python.org/zh-cn/3/library/fractions.html#fractions.Fraction)。Python 也内置对复数的支持，使用后缀 j 或者 J 就可以表示虚数部分（例如 3+5j ）。

练习

>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # division always returns a floating point number
1.6
>>> 17 / 3  # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2  # result * divisor + remainder
17

等号 (=) 用于给一个变量赋值。然后在下一个交互提示符之前不会有结果显示出来:

>>> width = 20
>>> height = 5 * 9
>>> width * height
900

如果一个变量未定义（未赋值），试图使用它时会向你提示错误:

>>> n  # try to access an undefined variable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

Python中提供浮点数的完整支持；包含多种混合类型运算数的运算会把整数转换为浮点数:
在交互模式下，上一次打印出来的表达式被赋值给变量 _。这意味着当你把Python用作桌面计算器时，继续计算会相对简单，比如:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

字符串

Python 也可以操作字符串。字符串有多种形式，可以使用单引号（'...'），双引号（"..."）都可以获得同样的结果。

>>> name = "小明"
>>> name
'小明'
>>> name = '小明'
>>> name
'小明'
>>> name = '小明)))****77777%%%%%%'
>>> name
'小明)))****77777%%%%%%'
>>> name = '小明说：'我爱学习''
  File "<stdin>", line 1
    name = '小明说：'我爱学习''
                 ^
SyntaxError: invalid syntax
>>> name ="小明说：'我爱学习'"
>>> name
"小明说：'我爱学习'"
>>> name = '小明说：\'我爱学习\''    #  \ 表示转义， \' 单引号当作普通字符串对待。

在交互式解释器中，输出的字符串外面会加上引号，特殊字符会使用反斜杠来转义。虽然有时这看起来会与输入不一样（外面所加的引号可能会改变），但两个字符串是相同的。如果字符串中有单引号而没有双引号，该字符串外将加双引号来表示，否则就加单引号。 [print()](https://docs.python.org/zh-cn/3/library/functions.html#print) 函数会生成可读性更强的输出，即略去两边的引号，并且打印出经过转义的特殊字符:

>>> '"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.

如果你不希望前置了 \ 的字符转义成特殊字符，可以使用 原始字符串 方式，在引号前添加 r 即可:

>>> 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

字符串运算符

+运算符
+支支持字符串+字符串

>>> 'xiaoming' + "xiaohong"
'xiaomingxiaohong'

不能与其它类型数据相加.

>>> 'xiaoming' + "xiaohong" + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

运算
字符串只能与数字（int）相乘

>>> "xiaoming"*10
'xiaomingxiaomingxiaomingxiaomingxiaomingxiaomingxiaomingxiaomingxiaomingxiaoming'
>>> "xiaoming"*"10"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't multiply sequence by non-int of type 'str'
>>> "xiaoming"*2.3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't multiply sequence by non-int of type 'float'

练习

>>> print("*"*30,"\n","西瓜"," "*15, "$10","\n","可乐"," "*15,"$4","\n"+"*"*30)
******************************
 西瓜                 $10
 可乐                 $4
******************************

字符串操作

len() 查看字符串的长度

>>> a = "abcdefg"
>>> a
'abcdefg'
>>> len(a)
7

索引

-6 -5 -4 -3 -2 -1 
|  |  |  |  |  |
P  y  t  h  o  n
|  |  |  |  |  |
0  1  2  3  4  5

>>> "Python"
'Python'
>>> "Python"[0]
'P'
>>> "Python"[1]
'y'
>>> "Python"[10]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: string index out of range
>>> "Python"[-1]
'n'
>>> "Python"[-2]
'o'
>>> "Python"[-3]
'h'
>>> "Python"[-100]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: string index out of range

切片

>>> "Python"[0:2]
'Py'
>>> "Python"[0:3]
'Pyt'
>>> "Python"[1:5]
'ytho'
>>> "Python"[-3:-1]
'ho'
>>> "Python"[-3:]
'hon'
>>> "Python"[:3]
'Pyt'
>>> "Python"[:]
'Python'

步进 step 步长

>>> w = "abcdefg"
>>> w[0:5:1]
'abcde'
>>> w[0:5:2]
'ace'
>>> w[0:5:3]
'ad'

步长的值为负数时，表示从右向左来取数据

>>> w[:5:1]
'abcde'
>>> w[:5:2]
'ace'
>>> w[-3:]
'efg'
>>> w[-3::2]
'eg'
>>> w[-3::-1]
'edcba'
>>> w[-1:-3:-1]
'gf'
>>> w[-1:-4:-1]
'gfe'
>>> w[::-1]
'gfedcba'

常用方法

>>> s = 'supercalifragilisticexpialidocious'

count() 统计某个字符在字符串中出现的次数
```
>>> s.count("s")
3
```

center(len,fill) 字符串满足长度，不够的部分用指定的字符串来填充。

>>> "coke".center(50,"*")
'***********************coke***********************'
>>> "coke".center(30,"*")
'*************coke*************'
>>> "coke".center(30,"=")
'=============coke============='

index() 查找某个字符在字符串中的索引位置。

>>> s.index("a")
6
>>> s.index("a",6)
6
>>> s.index("a",7)
11
>>> s.index("a",12)
24
>>> s.index("a",25)   #如果没有这个值，则报错
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found

find() 找字符索引，如果没有找到，返回结果 -1

>>> s.find('a')
6
>>> s.find('a',7)
11
>>> s.find('a',12)
24
>>> s.find('a',25)
-1

upper() 转换大写

>>> s.upper()
'SUPERCALIFRAGILISTICEXPIALIDOCIOUS'

lower() 转化小写
```
>>> "HelloWorld".lower()
'helloworld'
```

strip() 默认去除两边空格，加上参数，去除指定字符

>>> '      s '.strip()
's'
>>> s
'supercalifragilisticexpialidocious'
>>> s.strip('s')
'upercalifragilisticexpialidociou'

replace() 替换指定字符串

>>> s.replace('s','xiaoming')
'xiaomingupercalifragilixiaomingticexpialidociouxiaoming'
>>> s
'supercalifragilisticexpialidocious'
>>> s.replace('s','xiaoming',2)
'xiaomingupercalifragilixiaomingticexpialidocious'
>>> s.replace('s','xiaoming',-2)
'xiaomingupercalifragilixiaomingticexpialidociouxiaoming'
>>> s[::-1].replace('s','xiaoming',2)
'xiaominguoicodilaipxecitxiaomingiligarfilacrepus'
>>> s[::-1].replace('s','xiaoming',2)[::-1]
'supercalifragilignimoaixticexpialidociougnimoaix'
>>> s[::-1].replace('s','xiaoming',2)[::-1].replace("gnimoaix",'xiaoming') # 最后两个s 替换为xiaoming
'supercalifragilixiaomingticexpialidociouxiaoming'

字符串格式化

format 格式化

>>> "my name is {} ".format("xiaoming")
'my name is xiaoming '
>>> "my name is {} ".format("xiaowang")
'my name is xiaowang '
>>> "my name is {}, my age is {} ".format("xiaowang",20)
'my name is xiaowang, my age is 20 '
>>> "my name is {}, my age is {}, my sex is {} ".format("xiaowang",20,"nan")
'my name is xiaowang, my age is 20, my sex is nan '
>>> "my name is {name}, my age is {age}, my sex is {sex} ".format(age=20,sex="nan",name="xiaoming")
'my name is xiaoming, my age is 20, my sex is nan '
>>> "my name is {0}, my age is {1}, my sex is {2} ".format("xiaoming",20,"nan")
'my name is xiaoming, my age is 20, my sex is nan '
>>> "my name is {0}, my age is {2}, my sex is {1} ".format("xiaoming",20,"nan")
'my name is xiaoming, my age is nan, my sex is 20 '
>>> "my name is {0}, my age is {2}, my sex is {1} ".format("xiaoming","nan",20)
'my name is xiaoming, my age is 20, my sex is nan '

f 格式化

>>> name = "xiaoming"
>>> age =10
>>> f"my name is {name}, my age is {age}"

【附录】

官网教程 https://docs.python.org/zh-cn/3/tutorial/index.html 经典学习手册
https://docs.python.org/zh-cn/3/tutorial/introduction.html
https://docs.python.org/zh-cn/3/library/stdtypes.html#string-methods