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‎feature.md‎

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@@ -41,18 +41,50 @@ Python小例子 https://github.com/jackzhenguo/python-small-examples
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### 一、 数字
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| ID | 链接 | 难度 |
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| ---- | --------------------------- | ---- |
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| 1 | [求绝对值](md/1.md) | |
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| 2 | [进制转化](md/2.md) | |
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| 3 | [整数和ASCII互转](md/3.md) | |
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| 4 | [元素都为真检查](md/4.md) | |
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| 5 | [至少一个为真检查](md/5.md) | |
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| 6 | [判断是真是假](md/6.md) | |
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| 7 | [创建复数](md/7.md) | |
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| 8 | [取商和余数](md/8.md) | |
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| 9 | [转为浮点类型](md/9.md) | |
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| 10 | [转为整型](md/10.md) | |
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| 11 | [次幂](md/11.md) | |
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| 12 | [四舍五入](md/12.md) | |
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| 13 | [链式比较](md/13.md) | |
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| ID | md文件链接 | 难度 |
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| ---- | ---------------------------------- | ---- |
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| 1 | [求绝对值](md/1.md) | |
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| 2 | [进制转化](md/2.md) | |
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| 3 | [整数和ASCII互转](md/3.md) | |
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| 4 | [元素都为真检查](md/4.md) | |
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| 5 | [至少一个为真检查](md/5.md) | |
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| 6 | [判断是真是假](md/6.md) | |
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| 7 | [创建复数](md/7.md) | |
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| 8 | [取商和余数](md/8.md) | |
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| 9 | [转为浮点类型](md/9.md) | |
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| 10 | [转为整型](md/10.md) | |
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| 11 | [次幂](md/11.md) | |
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| 12 | [四舍五入](md/12.md) | |
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| 13 | [链式比较](md/13.md) | |
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| 14 | [字符串转字节](md/14.md) | |
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| 15 | [任意对象转为字符串](md/15.md) | |
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| 16 | [执行字符串表示的代码](md/16.md) | |
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| 17 | [计算表达式](md/17.md) | |
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| 18 | [字符串格式化](md/18.md) | |
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| 19 | [拿来就用的排序函数](md/19.md) | |
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| 20 | [求和函数](md/20.md) | |
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| 21 | [nonlocal用于内嵌函数中](md/21.md) | |
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| 22 | [global 声明全局变量](md/22.md) | |
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| 23 | [交换两元素](md/23.md) | |
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| 24 | [操作函数对象](md/24.md) | |
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| 25 | [生成逆序序列](md/25.md) | |
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| 26 | [函数的五类参数使用例子](md/26.md) | |
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| 27 | [使用slice对象](md/27.md) | |
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| 28 | [lambda 函数的动画演示](md/28.md) | |
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| 29 | [转为字典](md/29.md) | |
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| 30 | [冻结集合](md/30.md) | |
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| 31 | [转为集合类型](md/31.md) | |
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| 32 | [转元组](md/32.md) | |
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| 33 | [对象是否可调用](md/33.md) | |
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| 34 | [ascii 展示对象](md/34.md) | |
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| 35 | [类方法](md/35.md) | |
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| 36 | [动态删除属性](md/36.md) | |
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| 37 | [一键查看对象所有方法](md/37.md) | |
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| 38 | [动态获取对象属性](md/38.md) | |
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| 39 | [对象是否有某个属性](md/39.md) | |
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| 40 | [对象门牌号](md/40.md) | |
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| 41 | [isinstance](md/41.md) | |
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| 42 | [issubclass父子关系鉴定](md/42.md) | |
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| 43 | [所有对象之根](md/43.md) | |
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| 44 | [创建属性的两种方法](md/44.md) | |
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| 45 | [查看对象类型](md/45.md) | |

‎md/14.md‎

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#### 14 字符串转字节　　
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字符串转换为字节类型
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```python
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In [12]: s = "apple"
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In [13]: bytes(s,encoding='utf-8')
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Out[13]: b'apple'
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```

‎md/15.md‎

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#### 15 任意对象转为字符串　　
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```python
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In [14]: i = 100
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In [15]: str(i)
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Out[15]: '100'
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In [16]: str([])
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Out[16]: '[]'
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In [17]: str(tuple())
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Out[17]: '()'
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```

‎md/16.md‎

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#### 16 执行字符串表示的代码
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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。
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```python
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In [1]: s = "print('helloworld')"
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In [2]: r = compile(s,"<string>", "exec")
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In [3]: r
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Out[3]: <code object <module> at 0x0000000005DE75D0, file "<string>", line 1>
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In [4]: exec(r)
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helloworld
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```

‎md/17.md‎

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#### 17 计算表达式
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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
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```python
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In [1]: s = "1 + 3 +5"
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...: eval(s)
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...:
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Out[1]: 9
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```

‎md/18.md‎

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#### 18 字符串格式化　
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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。
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```
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In [104]: print("i am {0},age{1}".format("tom",18))
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i am tom,age18
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```
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| 3.1415926 | {:.2f} | 3.14 | 保留小数点后两位 |
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| ---------- | ------- | --------- | ---------------------------- |
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| 3.1415926 | {:+.2f} | +3.14 | 带符号保留小数点后两位 |
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| -1 | {:+.2f} | -1.00 | 带符号保留小数点后两位 |
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| 2.71828 | {:.0f} | 3 | 不带小数 |
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| 5 | {:0>2d} | 05 | 数字补零 (填充左边, 宽度为2) |
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| 5 | {:x<4d} | 5xxx | 数字补x (填充右边, 宽度为4) |
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| 10 | {:x<4d} | 10xx | 数字补x (填充右边, 宽度为4) |
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| 1000000 | {:,} | 1,000,000 | 以逗号分隔的数字格式 |
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| 0.25 | {:.2%} | 25.00% | 百分比格式 |
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| 1000000000 | {:.2e} | 1.00e+09 | 指数记法 |
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| 18 | {:>10d} | ' 18' | 右对齐 (默认, 宽度为10) |
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| 18 | {:<10d} | '18 ' | 左对齐 (宽度为10) |
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| 18 | {:^10d} | ' 18 ' | 中间对齐 (宽度为10) |

‎md/19.md‎

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#### 19 拿来就用的排序函数
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排序：
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```python
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In [1]: a = [1,4,2,3,1]
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In [2]: sorted(a,reverse=True)
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Out[2]: [4, 3, 2, 1, 1]
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In [3]: a = [{'name':'xiaoming','age':18,'gender':'male'},{'name':'
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...: xiaohong','age':20,'gender':'female'}]
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In [4]: sorted(a,key=lambda x: x['age'],reverse=False)
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Out[4]:
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[{'name': 'xiaoming', 'age': 18, 'gender': 'male'},
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{'name': 'xiaohong', 'age': 20, 'gender': 'female'}]
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```

‎md/20.md‎

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#### 20 求和函数
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求和：
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```python
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In [181]: a = [1,4,2,3,1]
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In [182]: sum(a)
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Out[182]: 11
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In [185]: sum(a,10) #求和的初始值为10
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Out[185]: 21
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```

‎md/21.md‎

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#### 21 nonlocal用于内嵌函数中
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关键词`nonlocal`常用于函数嵌套中，声明变量`i`为非局部变量；
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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。
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```python
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def excepter(f):
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i = 0
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t1 = time.time()
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def wrapper():
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try:
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f()
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except Exception as e:
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nonlocal i
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i += 1
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print(f'{e.args[0]}: {i}')
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t2 = time.time()
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if i == n:
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print(f'spending time:{round(t2-t1,2)}')
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return wrapper
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```

‎md/22.md‎

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#### 22 global 声明全局变量
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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：
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```python
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i = 5
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def f():
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print(i)
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def g():
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print(i)
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pass
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f()
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g()
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```
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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`.
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但是，如果我想要有个函数对`i`递增，这样：
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```python
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def h():
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i += 1
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h()
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```
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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。
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`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：
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```python
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i = 0
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def h():
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global i
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i += 1
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h()
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print(i)
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```

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