在Python 3.x中增加了nonlocal关键字，关于变量的声明，引自官方文档：

Assignment of an object to a single target is recursively defined as follows.

If the target is an identifier (name):

• If the name does not occur in a global or nonlocal statement in the current code block: the name is bound to the object in the current local namespace.

• Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by nonlocal, respectively.

在当前作用域/代码块中，一个变量如果没有用关键字global或者nonlocal声明，则是局部变量，否则它是一个全局（global）变量 或者是外层作用域的非局部（nonlocal）变量。

python中引用变量的顺序： 当前作用域局部变量->外层作用域变量->当前模块中的全局变量->python内置变量

注：如果只是引用全局变量或者外部变量，而不改变值的话，可以不加关键字，如果要改变值，则需要用关键字声明，否则会抛出error

example：

1 g_count = 5
2 
3 def test1():
4     x = g_count + 5
5     return x
6 
7 print(test1()) # 输出 10

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1 g_count = 5
2 
3 def test1():
4     #global g_count  不用global声明，会出现错误
5     g_count += 2
6     return ｇ_count
7 
8 print(test1())

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UnboundLocalError: local variable ‘g_count‘ referenced before assignment，局部变量在定义之前被引用。

noncal关键字的使用

example：

 1 def test3():
 2     counter = 0
 3     def sub_test():
 4         nonlocal counter
 5         counter += 2
 6         return counter
 7     return sub_test
 8 
 9 
10 f = test3()
11 print(f())  # ==>2
12 print(f())  # ==>4
13 print(f())  # ==>6

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Python 关键字之 nonlocal