1 def print_msg(msg):
2 # This is the outer enclosing function
3
4 def printer(): 5 # This is the nested function 6 print(msg) 7 8 printer() 9 10 # We execute the function 11 # Output: Hello 12 print_msg("Hello")
如上所示，printer是内嵌函数（nested function），根据LEGB的E原则，取到了传入的msg参数
def print_msg(msg):
# This is the outer enclosing function
def printer():
# This is the nested function
print(msg)
return printer # this got changed
# Now let's try calling this function.
# Output: Hello
another = print_msg("Hello")
another()
如果将print_msg内的printer调用替换成将其返回，赋值到变量another，print another会得到 ： <function printer at 0x02666AB0>， 可见赋值给another的是执行后的print_msg的值，即 printer，那么another()即代表执行printer()，而此时printer()仍然可以记得msg的值，即使print_msg函数已经结束执行，或者在another=print_msg("hello")后将print_msg删除，del print_msg也无妨。
闭包必须包含：
- We must have a nested function (function inside a function). 一个嵌套函数
- The nested function must refer to a value defined in the enclosing function. 嵌套函数必须引用一个外部定义的变量
- The enclosing function must return the nested function. enclosing函数返回嵌套函数
__closure__
closure属性返回一个包含cell的元组，里面每个cell保存了作用域变量
print another.__closure__
>>> (<cell at 0x024FC990: str object at 0x024FC960>,)
print another.__closure__[0].cell_contents
>>>Hello
参考：https://www.programiz.com/python-programming/closure