223个Python小例子（1-60）

知行编程网 2022-01-04 18:11 知行编程网 | 隐藏边栏 | 抢沙发 | 4 0

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Python小例子：https://github.com/jackzhenguo/python-small-examples

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小例子

一、数字

1 求绝对值

绝对值或复数的模

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">abs</span>(<span style="color: #005cc5;">-</span><span style="color: #005cc5;">6</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">6</span>

2 进制转化

十进制转换为二进制：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">2</span>]: <span style="color: #6f42c1;">bin</span>(<span style="color: #005cc5;">10</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">2</span>]: <span style="color: #032f62;">'0b1010'</span>

十进制转换为八进制：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">3</span>]: <span style="color: #6f42c1;">oct</span>(<span style="color: #005cc5;">9</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">3</span>]: <span style="color: #032f62;">'0o11'</span>

十进制转换为十六进制：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">4</span>]: <span style="color: #6f42c1;">hex</span>(<span style="color: #005cc5;">15</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">4</span>]: <span style="color: #032f62;">'0xf'</span>

3 整数和ASCII互转

十进制整数对应的ASCII字符

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">chr</span>(<span style="color: #005cc5;">65</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #032f62;">'A'</span>

查看某个ASCII字符对应的十进制数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">ord</span>(<span style="color: #032f62;">'A'</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">65</span>

4 元素都为真检查

所有元素都为真，返回 True，否则为False

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">5</span>]: <span style="color: #6f42c1;">all</span>([<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">6</span>])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">5</span>]: <span style="color: #005cc5;">False</span>

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">6</span>]: <span style="color: #6f42c1;">all</span>([<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">6</span>]: <span style="color: #005cc5;">True</span>

5 元素至少一个为真检查

至少有一个元素为真返回True，否则False

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">7</span>]: <span style="color: #6f42c1;">any</span>([<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">0</span>,[]])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">7</span>]: <span style="color: #005cc5;">False</span>

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">8</span>]: <span style="color: #6f42c1;">any</span>([<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">1</span>])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">8</span>]: <span style="color: #005cc5;">True</span>

6 判断是真是假

测试一个对象是True, 还是False.

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">9</span>]: <span style="color: #6f42c1;">bool</span>([<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">0</span>])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">9</span>]: <span style="color: #005cc5;">True</span>

In [10]: bool([])
Out[10]: False

In [11]: bool([1,0,1])
Out[11]: True

7 创建复数

创建一个复数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">complex</span>(<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">2</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: (<span style="color: #005cc5;">1</span><span style="color: #005cc5;">+</span><span style="color: #005cc5;">2j</span>)

8 取商和余数

分别取商和余数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">divmod</span>(<span style="color: #005cc5;">10</span>,<span style="color: #005cc5;">3</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: (<span style="color: #005cc5;">3</span>, <span style="color: #005cc5;">1</span>)

9 转为浮点类型

将一个整数或数值型字符串转换为浮点数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">float</span>(<span style="color: #005cc5;">3</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">3.0</span>

如果不能转化为浮点数，则会报ValueError:

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">2</span>]: <span style="color: #6f42c1;">float</span>(<span style="color: #032f62;">'a'</span>)
<span style="color: #6a737d;"># ValueError: could not convert string to float: 'a'</span>

10 转为整型

int(x, base =10) , x可能为字符串或数值，将x 转换为一个普通整数。如果参数是字符串，那么它可能包含符号和小数点。如果超出了普通整数的表示范围，一个长整数被返回。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">int</span>(<span style="color: #032f62;">'12'</span>,<span style="color: #005cc5;">16</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">18</span>

11 次幂

base为底的exp次幂，如果mod给出，取余

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">pow</span>(<span style="color: #005cc5;">3</span>, <span style="color: #005cc5;">2</span>, <span style="color: #005cc5;">4</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">1</span>

12 四舍五入

四舍五入，ndigits代表小数点后保留几位：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">11</span>]: <span style="color: #6f42c1;">round</span>(<span style="color: #005cc5;">10.0222222</span>, <span style="color: #005cc5;">3</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">11</span>]: <span style="color: #005cc5;">10.022</span>

In [12]: round(10.05,1)
Out[12]: 10.1

13 链式比较

i <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">3</span>
<span style="color: #6f42c1;">print</span>(<span style="color: #005cc5;">1</span> <span style="color: #005cc5;"><</span> i <span style="color: #005cc5;"><</span> <span style="color: #005cc5;">3</span>)  <span style="color: #6a737d;"># False</span>
<span style="color: #6f42c1;">print</span>(<span style="color: #005cc5;">1</span> <span style="color: #005cc5;"><</span> i <span style="color: #005cc5;"><=</span> <span style="color: #005cc5;">3</span>)  <span style="color: #6a737d;"># True</span>

二、字符串

14 字符串转字节

字符串转换为字节类型

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">12</span>]: s <span style="color: #005cc5;">=</span> <span style="color: #032f62;">"apple"</span>

In [13]: bytes(s,encoding='utf-8')
Out[13]: b'apple'

15 任意对象转为字符串

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">14</span>]: i <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">100</span>

In [15]: str(i)
Out[15]: '100'

In [16]: str([])
Out[16]: '[]'

In [17]: str(tuple())
Out[17]: '()'

16 执行字符串表示的代码

将字符串编译成python能识别或可执行的代码，也可以将文字读成字符串再编译。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: s  <span style="color: #005cc5;">=</span> <span style="color: #032f62;">"print('helloworld')"</span>

In [2]: r = compile(s,"<string>", "exec")

In [3]: r
Out[3]: <code object <module> at 0x0000000005DE75D0, file "<string>", line 1>

In [4]: exec(r)
helloworld

17 计算表达式

将字符串str 当成有效的表达式来求值并返回计算结果取出字符串中内容

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: s <span style="color: #005cc5;">=</span> <span style="color: #032f62;">"1 + 3 +5"</span>
    ...: <span style="color: #6f42c1;">eval</span>(s)
    ...:
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">9</span>

18 字符串格式化

格式化输出字符串，format(value, format_spec)实质上是调用了value的__format__(format_spec)方法。

In [104]: print("i am {0},age{1}".format("tom",18))
i am tom,age18

3.1415926	{:.2f}	3.14	保留小数点后两位
3.1415926	{:+.2f}	+3.14	带符号保留小数点后两位
-1	{:+.2f}	-1.00	带符号保留小数点后两位
2.71828	{:.0f}	3	不带小数
5	{:0>2d}	05	数字补零 (填充左边, 宽度为2)
5	{:x<4d}	5xxx	数字补x (填充右边, 宽度为4)
10	{:x<4d}	10xx	数字补x (填充右边, 宽度为4)
1000000	{:,}	1,000,000	以逗号分隔的数字格式
0.25	{:.2%}	25.00%	百分比格式
1000000000	{:.2e}	1.00e+09	指数记法
18	{:>10d}	' 18'	右对齐 (默认, 宽度为10)
18	{:<10d}	'18 '	左对齐 (宽度为10)
18	{:^10d}	' 18 '	中间对齐 (宽度为10)

三、函数

19 拿来就用的排序函数

排序：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: a <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">1</span>]

In [2]: sorted(a,reverse=True)
Out[2]: [4, 3, 2, 1, 1]

In [3]: a = [{'name':'xiaoming','age':18,'gender':'male'},{'name':'
...: xiaohong','age':20,'gender':'female'}]
In [4]: sorted(a,key=lambda x: x['age'],reverse=False)
Out[4]:
[{'name': 'xiaoming', 'age': 18, 'gender': 'male'},
{'name': 'xiaohong', 'age': 20, 'gender': 'female'}]

20 求和函数

求和：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">181</span>]: a <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">1</span>]

In [182]: sum(a)
Out[182]: 11

In [185]: sum(a,10) #求和的初始值为10
Out[185]: 21

21 nonlocal用于内嵌函数中

关键词nonlocal常用于函数嵌套中，声明变量i为非局部变量；如果不声明，i+=1表明i为函数wrapper内的局部变量，因为在i+=1引用(reference)时,i未被声明，所以会报unreferenced variable的错误。

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">excepter</span>(f):
    i <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">0</span>
    t1 <span style="color: #005cc5;">=</span> time.<span style="color: #6f42c1;">time</span>()
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">wrapper</span>():
        <span style="color: #d73a49;">try</span>:
            <span style="color: #6f42c1;">f</span>()
        <span style="color: #d73a49;">except</span> <span style="color: #e36209;">Exception</span> <span style="color: #d73a49;">as</span> e:
            <span style="color: #d73a49;">nonlocal</span> i
            i <span style="color: #005cc5;">+=</span> <span style="color: #005cc5;">1</span>
            <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'<span style="color: #24292e;">{e.args[<span style="color: #005cc5;">0</span>]}</span>: <span style="color: #24292e;">{i}</span>'</span>)
            t2 <span style="color: #005cc5;">=</span> time.<span style="color: #6f42c1;">time</span>()
            <span style="color: #d73a49;">if</span> i <span style="color: #005cc5;">==</span> n:
                <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'spending time:<span style="color: #24292e;">{<span style="color: #6f42c1;">round</span>(t2<span style="color: #005cc5;">-</span>t1,<span style="color: #005cc5;">2</span>)}</span>'</span>)
    <span style="color: #d73a49;">return</span> wrapper

22 global 声明全局变量

先回答为什么要有global，一个变量被多个函数引用，想让全局变量被所有函数共享。有的伙伴可能会想这还不简单，这样写：

i <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">5</span>
<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">f</span>():
    <span style="color: #6f42c1;">print</span>(i)

def g():
print(i)
pass

f()
g()

f和g两个函数都能共享变量i，程序没有报错，所以他们依然不明白为什么要用global.

但是，如果我想要有个函数对i递增，这样：

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">h</span>():
    i <span style="color: #005cc5;">+=</span> <span style="color: #005cc5;">1</span>

h()

此时执行程序，bang, 出错了！抛出异常：UnboundLocalError，原来编译器在解释i+=1时会把i解析为函数h()内的局部变量，很显然在此函数内，编译器找不到对变量i的定义，所以会报错。

global就是为解决此问题而被提出，在函数h内，显式地告诉编译器i为全局变量，然后编译器会在函数外面寻找i的定义，执行完i+=1后，i还为全局变量，值加1：

i <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">0</span>
<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">h</span>():
    <span style="color: #d73a49;">global</span> i
    i <span style="color: #005cc5;">+=</span> <span style="color: #005cc5;">1</span>

h()
print(i)

23 交换两元素

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">swap</span>(a, b):
    <span style="color: #d73a49;">return</span> b, a

print(swap(1, 0)) # (0,1)

24 操作函数对象

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">31</span>]: def <span style="color: #6f42c1;">f</span>():
    ...:     <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">'i'm f'</span>)
    ...:

In [32]: def g():
...: print('i'm g')
...:

In [33]: [f,g][1]()
i'm g

创建函数对象的list，根据想要调用的index，方便统一调用。

25 生成逆序序列

<span style="color: #6f42c1;">list</span>(<span style="color: #6f42c1;">range</span>(<span style="color: #005cc5;">10</span>,<span style="color: #005cc5;">-</span><span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">-</span><span style="color: #005cc5;">1</span>)) <span style="color: #6a737d;"># [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]</span>

第三个参数为负时，表示从第一个参数开始递减，终止到第二个参数(不包括此边界)

26 函数的五类参数使用例子

python五类参数：位置参数，关键字参数，默认参数，可变位置或关键字参数的使用。

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">f</span>(a,<span style="color: #005cc5;">*</span>b,c<span style="color: #005cc5;">=</span><span style="color: #005cc5;">10</span>,<span style="color: #005cc5;">**</span>d):
  <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'a:<span style="color: #24292e;">{a}</span>,b:<span style="color: #24292e;">{b}</span>,c:<span style="color: #24292e;">{c}</span>,d:<span style="color: #24292e;">{d}</span>'</span>)

默认参数c不能位于可变关键字参数d后.

调用f:

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">10</span>]: <span style="color: #6f42c1;">f</span>(<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">5</span>,width<span style="color: #005cc5;">=</span><span style="color: #005cc5;">10</span>,height<span style="color: #005cc5;">=</span><span style="color: #005cc5;">20</span>)
a:<span style="color: #005cc5;">1</span>,b:(<span style="color: #005cc5;">2</span>, <span style="color: #005cc5;">5</span>),c:<span style="color: #005cc5;">10</span>,d:{<span style="color: #032f62;">'width'</span>: <span style="color: #005cc5;">10</span>, <span style="color: #032f62;">'height'</span>: <span style="color: #005cc5;">20</span>}

可变位置参数b实参后被解析为元组(2,5);而c取得默认值10; d被解析为字典.

再次调用f:

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">11</span>]: <span style="color: #6f42c1;">f</span>(a<span style="color: #005cc5;">=</span><span style="color: #005cc5;">1</span>,c<span style="color: #005cc5;">=</span><span style="color: #005cc5;">12</span>)
a:<span style="color: #005cc5;">1</span>,b:(),c:<span style="color: #005cc5;">12</span>,d:{}

a=1传入时a就是关键字参数，b,d都未传值，c被传入12，而非默认值。

注意观察参数a, 既可以f(1),也可以f(a=1) 其可读性比第一种更好，建议使用f(a=1)。如果要强制使用f(a=1)，需要在前面添加一个星号:

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">f</span>(*,a,<span style="color: #005cc5;">**</span>b):
  <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'a:<span style="color: #24292e;">{a}</span>,b:<span style="color: #24292e;">{b}</span>'</span>)

此时f(1)调用，将会报错：TypeError: f() takes 0 positional arguments but 1 was given

只能f(a=1)才能OK.

说明前面的*发挥作用，它变为只能传入关键字参数，那么如何查看这个参数的类型呢？借助python的inspect模块：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">22</span>]: for name,val <span style="color: #005cc5;">in</span> <span style="color: #6f42c1;">signature</span>(f).parameters.<span style="color: #6f42c1;">items</span>():
    ...:     <span style="color: #6f42c1;">print</span>(name,val.kind)
    ...:
a <span style="color: #e36209;">KEYWORD_ONLY</span>
b <span style="color: #e36209;">VAR_KEYWORD</span>

可看到参数a的类型为KEYWORD_ONLY，也就是仅仅为关键字参数。

但是，如果f定义为：

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">f</span>(a,<span style="color: #005cc5;">*</span>b):
  <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'a:<span style="color: #24292e;">{a}</span>,b:<span style="color: #24292e;">{b}</span>'</span>)

查看参数类型：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">24</span>]: for name,val <span style="color: #005cc5;">in</span> <span style="color: #6f42c1;">signature</span>(f).parameters.<span style="color: #6f42c1;">items</span>():
    ...:     <span style="color: #6f42c1;">print</span>(name,val.kind)
    ...:
a <span style="color: #e36209;">POSITIONAL_OR_KEYWORD</span>
b <span style="color: #e36209;">VAR_POSITIONAL</span>

可以看到参数a既可以是位置参数也可是关键字参数。

27 使用slice对象

生成关于蛋糕的序列cake1：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: cake1 <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">list</span>(<span style="color: #6f42c1;">range</span>(<span style="color: #005cc5;">5</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">-</span><span style="color: #005cc5;">1</span>))

In [2]: b = cake1[1:10:2]

In [3]: b
Out[3]: [4, 2]

In [4]: cake1
Out[4]: [5, 4, 3, 2, 1]

再生成一个序列：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">5</span>]: from random <span style="color: #d73a49;">import</span> randint
   ...: cake2 <span style="color: #005cc5;">=</span> [<span style="color: #6f42c1;">randint</span>(<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">100</span>) <span style="color: #d73a49;">for</span> _ <span style="color: #005cc5;">in</span> <span style="color: #6f42c1;">range</span>(<span style="color: #005cc5;">100</span>)]
   ...: <span style="color: #6a737d;"># 同样以间隔为2切前10个元素，得到切片d</span>
   ...: d <span style="color: #005cc5;">=</span> cake2[<span style="color: #005cc5;">1</span>:<span style="color: #005cc5;">10</span>:<span style="color: #005cc5;">2</span>]
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">6</span>]: d
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">6</span>]: [<span style="color: #005cc5;">75</span>, <span style="color: #005cc5;">33</span>, <span style="color: #005cc5;">63</span>, <span style="color: #005cc5;">93</span>, <span style="color: #005cc5;">15</span>]

你看，我们使用同一种切法，分别切开两个蛋糕cake1,cake2. 后来发现这种切法极为经典，又拿它去切更多的容器对象。

那么，为什么不把这种切法封装为一个对象呢？于是就有了slice对象。

定义slice对象极为简单，如把上面的切法定义成slice对象：

perfect_cake_slice_way <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">slice</span>(<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">10</span>,<span style="color: #005cc5;">2</span>)
<span style="color: #6a737d;">#去切cake1</span>
cake1_slice <span style="color: #005cc5;">=</span> cake1[perfect_cake_slice_way]
cake2_slice <span style="color: #005cc5;">=</span> cake2[perfect_cake_slice_way]

In [11]: cake1_slice
Out[11]: [4, 2]

In [12]: cake2_slice
Out[12]: [75, 33, 63, 93, 15]

与上面的结果一致。

对于逆向序列切片，slice对象一样可行：

a <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">5</span>,<span style="color: #005cc5;">7</span>,<span style="color: #005cc5;">9</span>,<span style="color: #005cc5;">0</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">5</span>,<span style="color: #005cc5;">7</span>]
a_ <span style="color: #005cc5;">=</span> a[<span style="color: #005cc5;">5</span>:<span style="color: #005cc5;">1</span>:<span style="color: #005cc5;">-</span><span style="color: #005cc5;">1</span>]

named_slice = slice(5,1,-1)
a_slice = a[named_slice]

In [14]: a_
Out[14]: [0, 9, 7, 5]

In [15]: a_slice
Out[15]: [0, 9, 7, 5]

频繁使用同一切片的操作可使用slice对象抽出来，复用的同时还能提高代码可读性。

28 lambda 函数的动画演示

有些读者反映，lambda函数不太会用，问我能不能解释一下。

比如，下面求这个 lambda函数：

<span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">max_len</span>(<span style="color: #005cc5;">*</span>lists):
    <span style="color: #d73a49;">return</span> <span style="color: #6f42c1;">max</span>(<span style="color: #005cc5;">*</span>lists, key<span style="color: #005cc5;">=</span><span style="color: #d73a49;">lambda</span> v: <span style="color: #6f42c1;">len</span>(v))

有两点疑惑：

参数v的取值？
lambda函数有返回值吗？如果有，返回值是多少？

调用上面函数，求出以下三个最长的列表：

r <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">max_len</span>([<span style="color: #005cc5;">1</span>, <span style="color: #005cc5;">2</span>, <span style="color: #005cc5;">3</span>], [<span style="color: #005cc5;">4</span>, <span style="color: #005cc5;">5</span>, <span style="color: #005cc5;">6</span>, <span style="color: #005cc5;">7</span>], [<span style="color: #005cc5;">8</span>])
<span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'更长的列表是<span style="color: #24292e;">{r}</span>'</span>)

程序完整运行过程，动画演示如下：

结论：

参数v的可能取值为*lists，也就是 tuple 的一个元素。
lambda函数返回值，等于lambda v冒号后表达式的返回值。

四、数据结构

29 转为字典

创建数据字典

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">dict</span>()
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: {}

In [2]: dict(a='a',b='b')
Out[2]: {'a': 'a', 'b': 'b'}

In [3]: dict(zip(['a','b'],[1,2]))
Out[3]: {'a': 1, 'b': 2}

In [4]: dict([('a',1),('b',2)])
Out[4]: {'a': 1, 'b': 2}

30 冻结集合

创建一个不可修改的集合。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">frozenset</span>([<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>])
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">frozenset</span>({<span style="color: #005cc5;">1</span>, <span style="color: #005cc5;">2</span>, <span style="color: #005cc5;">3</span>})

因为不可修改，所以没有像set那样的add和pop方法

31 转为集合类型

返回一个set对象，集合内不允许有重复元素：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">159</span>]: a <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">1</span>]

In [160]: set(a)
Out[160]: {1, 2, 3, 4}

32 转为切片对象

class slice(start, stop[, step])

返回一个表示由 range(start, stop, step) 所指定索引集的 slice对象，它让代码可读性、可维护性变好。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: a <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">1</span>]

In [2]: my_slice_meaning = slice(0,5,2)

In [3]: a[my_slice_meaning]
Out[3]: [1, 2, 1]

33 转元组

tuple() 将对象转为一个不可变的序列类型

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">16</span>]: i_am_list <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">5</span>]
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">17</span>]: i_am_tuple <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">tuple</span>(i_am_list)
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">18</span>]: i_am_tuple
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">18</span>]: (<span style="color: #005cc5;">1</span>, <span style="color: #005cc5;">3</span>, <span style="color: #005cc5;">5</span>)

五、类和对象

34 是否可调用

检查对象是否可被调用

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">callable</span>(str)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">True</span>

In [2]: callable(int)
Out[2]: True

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">18</span>]: class <span style="color: #e36209;">Student</span>():
    ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
    ...:         self.id <span style="color: #005cc5;">=</span> id
    ...:         self.name <span style="color: #005cc5;">=</span> name
    ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
    ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name
    ...

In [19]: xiaoming = Student('001','xiaoming')

In [20]: callable(xiaoming)
Out[20]: False

如果能调用xiaoming(), 需要重写Student类的__call__方法：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
    ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
    ...:         self.id <span style="color: #005cc5;">=</span> id
    ...:         self.name <span style="color: #005cc5;">=</span> name
    ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
    ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name
    ...:     def <span style="color: #6f42c1;">__call__</span>(self):
    ...:         <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">'I can be called'</span>)
    ...:         <span style="color: #6f42c1;">print</span>(<span style="color: #032f62;">f'my name is <span style="color: #24292e;">{self.name}</span>'</span>)
    ...:

In [2]: t = Student('001','xiaoming')

In [3]: t()
I can be called
my name is xiaoming

35 ascii 展示对象

调用对象的 __repr__ 方法，获得该方法的返回值，如下例子返回值为字符串

<span style="color: #005cc5;">>></span><span style="color: #005cc5;">></span> <span style="color: #d73a49;">class</span> <span style="color: #e36209;">Student</span>():
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
        self.id <span style="color: #005cc5;">=</span> id
        self.name <span style="color: #005cc5;">=</span> name
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__repr__</span>(self):
        <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name

调用：

<span style="color: #005cc5;">>></span><span style="color: #005cc5;">></span> xiaoming <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>(id<span style="color: #005cc5;">=</span><span style="color: #032f62;">'1'</span>,name<span style="color: #005cc5;">=</span><span style="color: #032f62;">'xiaoming'</span>)
<span style="color: #005cc5;">>></span><span style="color: #005cc5;">></span> xiaoming
id <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">1</span>, name <span style="color: #005cc5;">=</span> xiaoming
<span style="color: #005cc5;">>></span><span style="color: #005cc5;">></span> <span style="color: #6f42c1;">ascii</span>(xiaoming)
<span style="color: #032f62;">'id = 1, name = xiaoming'</span>

36 类方法

classmethod 装饰器对应的函数不需要实例化，不需要 self 参数，但第一个参数需要是表示自身类的 cls 参数，可以来调用类的属性，类的方法，实例化对象等。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
    ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
    ...:         self.id <span style="color: #005cc5;">=</span> id
    ...:         self.name <span style="color: #005cc5;">=</span> name
    ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
    ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name
    ...:     @classmethod
    ...:     def <span style="color: #6f42c1;">f</span>(cls):
    ...:         <span style="color: #6f42c1;">print</span>(cls)

37 动态删除属性

删除对象的属性

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">delattr</span>(xiaoming,<span style="color: #032f62;">'id'</span>)

In [2]: hasattr(xiaoming,'id')
Out[2]: False

38 一键查看对象所有方法

不带参数时返回当前范围内的变量、方法和定义的类型列表；带参数时返回参数的属性，方法列表。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">96</span>]: <span style="color: #6f42c1;">dir</span>(xiaoming)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">96</span>]:
[<span style="color: #032f62;">'__class__'</span>,
 <span style="color: #032f62;">'__delattr__'</span>,
 <span style="color: #032f62;">'__dict__'</span>,
 <span style="color: #032f62;">'__dir__'</span>,
 <span style="color: #032f62;">'__doc__'</span>,
 <span style="color: #032f62;">'__eq__'</span>,
 <span style="color: #032f62;">'__format__'</span>,
 <span style="color: #032f62;">'__ge__'</span>,
 <span style="color: #032f62;">'__getattribute__'</span>,
 <span style="color: #032f62;">'__gt__'</span>,
 <span style="color: #032f62;">'__hash__'</span>,
 <span style="color: #032f62;">'__init__'</span>,
 <span style="color: #032f62;">'__init_subclass__'</span>,
 <span style="color: #032f62;">'__le__'</span>,
 <span style="color: #032f62;">'__lt__'</span>,
 <span style="color: #032f62;">'__module__'</span>,
 <span style="color: #032f62;">'__ne__'</span>,
 <span style="color: #032f62;">'__new__'</span>,
 <span style="color: #032f62;">'__reduce__'</span>,
 <span style="color: #032f62;">'__reduce_ex__'</span>,
 <span style="color: #032f62;">'__repr__'</span>,
 <span style="color: #032f62;">'__setattr__'</span>,
 <span style="color: #032f62;">'__sizeof__'</span>,
 <span style="color: #032f62;">'__str__'</span>,
 <span style="color: #032f62;">'__subclasshook__'</span>,
 <span style="color: #032f62;">'__weakref__'</span>,

'name']

39 动态获取对象属性

获取对象的属性

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
   ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
   ...:         self.id <span style="color: #005cc5;">=</span> id
   ...:         self.name <span style="color: #005cc5;">=</span> name
   ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
   ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: getattr(xiaoming,'name') # 获取xiaoming这个实例的name属性值
Out[3]: 'xiaoming'

40 对象是否有这个属性

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
   ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
   ...:         self.id <span style="color: #005cc5;">=</span> id
   ...:         self.name <span style="color: #005cc5;">=</span> name
   ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
   ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: hasattr(xiaoming,'name')
Out[3]: True

In [4]: hasattr(xiaoming,'address')
Out[4]: False

41 对象门牌号

返回对象的内存地址

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">id</span>(xiaoming)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">98234208</span>

42 isinstance

判断object是否为类classinfo的实例，是返回true

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
   ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
   ...:         self.id <span style="color: #005cc5;">=</span> id
   ...:         self.name <span style="color: #005cc5;">=</span> name
   ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
   ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')

In [3]: isinstance(xiaoming,Student)
Out[3]: True

43 父子关系鉴定

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #6f42c1;">undergraduate</span>(<span style="color: #e36209;">Student</span>):
    ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">studyClass</span>(self):
    ...:         pass
    ...:     def <span style="color: #6f42c1;">attendActivity</span>(self):
    ...:         pass

In [2]: issubclass(undergraduate,Student)
Out[2]: True

In [3]: issubclass(object,Student)
Out[3]: False

In [4]: issubclass(Student,object)
Out[4]: True

如果class是classinfo元组中某个元素的子类，也会返回True

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">issubclass</span>(int,(int,float))
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">True</span>

44 所有对象之根

object 是所有类的基类

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: o <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">object</span>()

In [2]: type(o)
Out[2]: object

45 创建属性的两种方式

返回 property 属性，典型的用法：

<span style="color: #d73a49;">class</span> <span style="color: #e36209;">C</span>:
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self):
        self._x <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">None</span>

def getx(self):
return self._x

def setx(self, value):
self._x = value

def delx(self):
del self._x
# 使用property类创建 property 属性
x = property(getx, setx, delx, "I'm the 'x' property.")

使用python装饰器，实现与上完全一样的效果代码：

<span style="color: #d73a49;">class</span> <span style="color: #e36209;">C</span>:
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self):
        self._x <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">None</span>

@property
def x(self):
return self._x

@x.setter
def x(self, value):
self._x = value

@x.deleter
def x(self):
del self._x

46 查看对象类型

class type(name, bases, dict)

传入一个参数时，返回 object 的类型：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">Student</span>():
   ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,id,name):
   ...:         self.id <span style="color: #005cc5;">=</span> id
   ...:         self.name <span style="color: #005cc5;">=</span> name
   ...:     def <span style="color: #6f42c1;">__repr__</span>(self):
   ...:         <span style="color: #d73a49;">return</span> <span style="color: #032f62;">'id = '</span><span style="color: #005cc5;">+</span>self.id <span style="color: #005cc5;">+</span><span style="color: #032f62;">', name = '</span><span style="color: #005cc5;">+</span>self.name
   ...:

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: type(xiaoming)
Out[3]: __main__.Student

In [4]: type(tuple())
Out[4]: tuple

47 元类

xiaoming, xiaohong, xiaozhang 都是学生，这类群体叫做 Student.

Python 定义类的常见方法，使用关键字 class

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">36</span>]: class <span style="color: #e36209;">Student</span>(object):
    ...:     pass

xiaoming, xiaohong, xiaozhang 是类的实例，则：

xiaoming <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>()
xiaohong <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>()
xiaozhang <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>()

创建后，xiaoming 的 __class__ 属性，返回的便是 Student类

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">38</span>]: xiaoming.__class__
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">38</span>]: __main__.<span style="color: #e36209;">Student</span>

问题在于，Student 类有 __class__属性，如果有，返回的又是什么？

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">39</span>]: xiaoming.__class__.__class__
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">39</span>]: type

哇，程序没报错，返回 type

那么，我们不妨猜测：Student 类，类型就是 type

换句话说，Student类就是一个对象，它的类型就是 type

所以，Python 中一切皆对象，类也是对象

Python 中，将描述 Student 类的类被称为：元类。

按照此逻辑延伸，描述元类的类被称为：元元类，开玩笑了~ 描述元类的类也被称为元类。

聪明的朋友会问了，既然 Student 类可创建实例，那么 type 类可创建实例吗？如果能，它创建的实例就叫：类了。你们真聪明！

说对了，type 类一定能创建实例，比如 Student 类了。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">40</span>]: <span style="color: #e36209;">Student</span> <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">type</span>(<span style="color: #032f62;">'Student'</span>,(),{})

In [41]: Student
Out[41]: __main__.Student

它与使用 class 关键字创建的 Student 类一模一样。

Python 的类，因为又是对象，所以和 xiaoming，xiaohong 对象操作相似。支持：

赋值
拷贝
添加属性
作为函数参数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">43</span>]: <span style="color: #e36209;">StudentMirror</span> <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span> <span style="color: #6a737d;"># 类直接赋值 # 类直接赋值</span>
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">44</span>]: <span style="color: #e36209;">Student</span>.class_property <span style="color: #005cc5;">=</span> <span style="color: #032f62;">'class_property'</span> <span style="color: #6a737d;"># 添加类属性</span>
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">46</span>]: <span style="color: #6f42c1;">hasattr</span>(<span style="color: #e36209;">Student</span>, <span style="color: #032f62;">'class_property'</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">46</span>]: <span style="color: #005cc5;">True</span>

元类，确实使用不是那么多，也许先了解这些，就能应付一些场合。就连 Python 界的领袖 Tim Peters 都说：

“元类就是深度的魔法，99%的用户应该根本不必为此操心。

六、工具

48 枚举对象

返回一个可以枚举的对象，该对象的next()方法将返回一个元组。

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: s <span style="color: #005cc5;">=</span> [<span style="color: #032f62;">"a"</span>,<span style="color: #032f62;">"b"</span>,<span style="color: #032f62;">"c"</span>]
    ...: for i ,v <span style="color: #005cc5;">in</span> <span style="color: #6f42c1;">enumerate</span>(s,<span style="color: #005cc5;">1</span>):
    ...:     <span style="color: #6f42c1;">print</span>(i,v)
    ...:
<span style="color: #005cc5;">1</span> a
<span style="color: #005cc5;">2</span> b
<span style="color: #005cc5;">3</span> c

49 查看变量所占字节数

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: import sys

In [2]: a = {'a':1,'b':2.0}

In [3]: sys.getsizeof(a) # 占用240个字节
Out[3]: 240

50 过滤器

在函数中设定过滤条件，迭代元素，保留返回值为True的元素：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: fil <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">filter</span>(<span style="color: #d73a49;">lambda</span> x: x<span style="color: #005cc5;">></span><span style="color: #005cc5;">10</span>,[<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">11</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">45</span>,<span style="color: #005cc5;">7</span>,<span style="color: #005cc5;">6</span>,<span style="color: #005cc5;">13</span>])

In [2]: list(fil)
Out[2]: [11, 45, 13]

51 返回对象的哈希值

返回对象的哈希值，值得注意的是自定义的实例都是可哈希的，list, dict, set等可变对象都是不可哈希的(unhashable)

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">hash</span>(xiaoming)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #005cc5;">6139638</span>

In [2]: hash([1,2,3])
# TypeError: unhashable type: 'list'

52 一键帮助

返回对象的帮助文档

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">help</span>(xiaoming)
<span style="color: #e36209;">Help</span> on <span style="color: #e36209;">Student</span> <span style="color: #005cc5;">in</span> module __main__ object:

53 获取用户输入　

获取用户输入内容

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">input</span>()
aa
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #032f62;">'aa'</span>

54 创建迭代器类型

使用iter(obj, sentinel), 返回一个可迭代对象, sentinel可省略(一旦迭代到此元素，立即终止)

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: lst <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">5</span>]

In [2]: for i in iter(lst):
...: print(i)
...:
1
3
5

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: class <span style="color: #e36209;">TestIter</span>(object):
    ...:     <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self):
    ...:         self.l<span style="color: #005cc5;">=</span>[<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">5</span>]
    ...:         self.i<span style="color: #005cc5;">=</span><span style="color: #6f42c1;">iter</span>(self.l)
    ...:     def <span style="color: #6f42c1;">__call__</span>(self):  <span style="color: #6a737d;">#定义了__call__方法的类的实例是可调用的</span>
    ...:         item <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">next</span>(self.i)
    ...:         <span style="color: #6f42c1;">print</span> (<span style="color: #032f62;">"__call__ is called,fowhich would return"</span>,item)
    ...:         return item
    ...:     def <span style="color: #6f42c1;">__iter__</span>(self): <span style="color: #6a737d;">#支持迭代协议(即定义有__iter__()函数)</span>
    ...:         <span style="color: #6f42c1;">print</span> (<span style="color: #032f62;">"__iter__ is called!!"</span>)
    ...:         <span style="color: #6f42c1;">return</span> iter(self.l)
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">2</span>]: t <span style="color: #005cc5;">=</span> <span style="color: #e36209;">TestIter</span>()
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">3</span>]: <span style="color: #6f42c1;">t</span>() <span style="color: #6a737d;"># 因为实现了__call__，所以t实例能被调用</span>
__call__ <span style="color: #005cc5;">is</span> called,which would <span style="color: #d73a49;">return</span> <span style="color: #005cc5;">1</span>
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">3</span>]: <span style="color: #005cc5;">1</span>

In [4]: for e in TestIter(): # 因为实现了__iter__方法，所以t能被迭代
...: print(e)
...:
__iter__ is called!!
1
3
2
3
4
5

55 打开文件

返回文件对象

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: fo <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">open</span>(<span style="color: #032f62;">'D:/a.txt'</span>,mode<span style="color: #005cc5;">=</span><span style="color: #032f62;">'r'</span>, encoding<span style="color: #005cc5;">=</span><span style="color: #032f62;">'utf-8'</span>)

In [2]: fo.read()
Out[2]: 'ufefflife is not so long,nI use Python to play.'

mode取值表：

字符	意义
`'r'`	读取（默认）
`'w'`	写入，并先截断文件
`'x'`	排它性创建，如果文件已存在则失败
`'a'`	写入，如果文件存在则在末尾追加
`'b'`	二进制模式
`'t'`	文本模式（默认）
`'+'`	打开用于更新（读取与写入）

56 创建range序列

range(stop)
range(start, stop[,step])

生成一个不可变序列：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">range</span>(<span style="color: #005cc5;">11</span>)
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">1</span>]: <span style="color: #6f42c1;">range</span>(<span style="color: #005cc5;">0</span>, <span style="color: #005cc5;">11</span>)

In [2]: range(0,11,1)
Out[2]: range(0, 11)

57 反向迭代器

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: rev <span style="color: #005cc5;">=</span> <span style="color: #6f42c1;">reversed</span>([<span style="color: #005cc5;">1</span>,<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">1</span>])

In [2]: for i in rev:
...: print(i)
...:
1
3
2
4
1

58 聚合迭代器

创建一个聚合了来自每个可迭代对象中的元素的迭代器：

<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">1</span>]: x <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">3</span>,<span style="color: #005cc5;">2</span>,<span style="color: #005cc5;">1</span>]
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">2</span>]: y <span style="color: #005cc5;">=</span> [<span style="color: #005cc5;">4</span>,<span style="color: #005cc5;">5</span>,<span style="color: #005cc5;">6</span>]
<span style="color: #e36209;">In</span> [<span style="color: #005cc5;">3</span>]: <span style="color: #6f42c1;">list</span>(<span style="color: #6f42c1;">zip</span>(y,x))
<span style="color: #e36209;">Out</span>[<span style="color: #005cc5;">3</span>]: [(<span style="color: #005cc5;">4</span>, <span style="color: #005cc5;">3</span>), (<span style="color: #005cc5;">5</span>, <span style="color: #005cc5;">2</span>), (<span style="color: #005cc5;">6</span>, <span style="color: #005cc5;">1</span>)]

In [4]: a = range(5)
In [5]: b = list('abcde')
In [6]: b
Out[6]: ['a', 'b', 'c', 'd', 'e']
In [7]: [str(y) + str(x) for x,y in zip(a,b)]
Out[7]: ['a0', 'b1', 'c2', 'd3', 'e4']

59 链式操作

<span style="color: #d73a49;">from</span> operator <span style="color: #d73a49;">import</span> (add, sub)

def add_or_sub(a, b, oper):
return (add if oper == '+' else sub)(a, b)

add_or_sub(1, 2, '-') # -1

60 对象序列化

对象序列化，是指将内存中的对象转化为可存储或传输的过程。很多场景，直接一个类对象，传输不方便。

但是，当对象序列化后，就会更加方便，因为约定俗成的，接口间的调用或者发起的 web 请求，一般使用 json 串传输。

实际使用中，一般对类对象序列化。先创建一个 Student 类型，并创建两个实例。

<span style="color: #d73a49;">class</span> <span style="color: #e36209;">Student</span>():
    <span style="color: #d73a49;">def</span> <span style="color: #6f42c1;">__init__</span>(self,<span style="color: #005cc5;">**</span>args):
        self.ids <span style="color: #005cc5;">=</span> args[<span style="color: #032f62;">'ids'</span>]
        self.name <span style="color: #005cc5;">=</span> args[<span style="color: #032f62;">'name'</span>]
        self.address <span style="color: #005cc5;">=</span> args[<span style="color: #032f62;">'address'</span>]
xiaoming <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>(ids <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">1</span>,name <span style="color: #005cc5;">=</span> <span style="color: #032f62;">'xiaoming'</span>,address <span style="color: #005cc5;">=</span> <span style="color: #032f62;">'北京'</span>)
xiaohong <span style="color: #005cc5;">=</span> <span style="color: #e36209;">Student</span>(ids <span style="color: #005cc5;">=</span> <span style="color: #005cc5;">2</span>,name <span style="color: #005cc5;">=</span> <span style="color: #032f62;">'xiaohong'</span>,address <span style="color: #005cc5;">=</span> <span style="color: #032f62;">'南京'</span>)

导入 json 模块，调用 dump 方法，就会将列表对象 [xiaoming,xiaohong]，序列化到文件 json.txt 中。

<span style="color: #d73a49;">import</span> json

with open('json.txt', 'w') as f:
json.dump([xiaoming,xiaohong], f, default=lambda obj: obj.__dict__, ensure_ascii=False, indent=2, sort_keys=True)

生成的文件内容，如下：

[
    {
        <span style="color: #032f62;">"address"</span>:<span style="color: #032f62;">"北京"</span>,
        <span style="color: #032f62;">"ids"</span>:<span style="color: #005cc5;">1</span>,
        <span style="color: #032f62;">"name"</span>:<span style="color: #032f62;">"xiaoming"</span>
    },
    {
        <span style="color: #032f62;">"address"</span>:<span style="color: #032f62;">"南京"</span>,
        <span style="color: #032f62;">"ids"</span>:<span style="color: #005cc5;">2</span>,
        <span style="color: #032f62;">"name"</span>:<span style="color: #032f62;">"xiaohong"</span>
    }
]

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223个Python小例子（1-60）

贡献

License

小例子

一、 数字

1 求绝对值

2 进制转化

3 整数和ASCII互转

4 元素都为真检查

5 元素至少一个为真检查

6 判断是真是假

7 创建复数

8 取商和余数

9 转为浮点类型

10 转为整型

11 次幂

12 四舍五入

13 链式比较

二、 字符串

14 字符串转字节

15 任意对象转为字符串

16 执行字符串表示的代码

17 计算表达式

18 字符串格式化

三、 函数

19 拿来就用的排序函数

20 求和函数

21 nonlocal用于内嵌函数中

22 global 声明全局变量

23 交换两元素

24 操作函数对象

25 生成逆序序列

26 函数的五类参数使用例子

27 使用slice对象

28 lambda 函数的动画演示

四、 数据结构

29 转为字典

30 冻结集合

31 转为集合类型

32 转为切片对象

33 转元组

五、 类和对象

34 是否可调用

35 ascii 展示对象

36 类方法

37 动态删除属性

38 一键查看对象所有方法

39 动态获取对象属性

40 对象是否有这个属性

41 对象门牌号

42 isinstance

43 父子关系鉴定

44 所有对象之根

45 创建属性的两种方式

46 查看对象类型

47 元类

六、工具

48 枚举对象

49 查看变量所占字节数

50 过滤器

51 返回对象的哈希值

52 一键帮助

53 获取用户输入

54 创建迭代器类型

55 打开文件

56 创建range序列

57 反向迭代器

58 聚合迭代器

59 链式操作

60 对象序列化

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一、数字

二、字符串

三、函数

四、数据结构

五、类和对象

53 获取用户输入　