What is a python closure? A brief introduction to python closures (with examples)

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What is closure?

In fact, when we use the function, we will trigger the closure inadvertently, because for some reason, the variables of the upper layer function will always be referenced or modified within the function, and then it will Triggering closure

So what is a closure? In fact, it is a rule that is triggered when functions are nested. This rule is triggered when the current function refers to a variable in the local namespace of the previous function and the function itself is returned as an object.
We say that the function that triggers the closure is called the closure function

The biggest feature of the closure is that it can be assigned to a variable after being returned by the outer function, and it carries the variables defined in the outer function

The example is as follows:

def func1():
    a = 2  #变量a为函数func1()开辟的局部命名空间内定义的变量
    def func2(x):
        return x**a  #函数fun2()内引用了变量a
    print('func2的id：',id(func2))
    return func2 #内层函数名被当作返回值，此时闭包规则达成。
res = func1() # 此时res等同于func2，我们可以看看他们的id完全相同
print('res的id：  ',id(res))
print(res(3))
del func1
print('删除函数func1之后：',res(5))
print(res.__closure__) 
print(type(res.__closure__[0])) 
print(res.__closure__[0].cell_contents) 
#此时，如果函数func2()没有引用外部函数变量a，我们可以试着把return x**a这句改成return x，这时函数func2就没有触发闭包规则
#此时如果再次打印res.__closure__，你会发现输出值为None

As can be seen from the output results below, even if the function func1 is deleted, the variable a that should disappear in theory can still be used (theoretically the local space corresponding to a disappears , then a should disappear). The reason why variable a can still be referenced is because of the closure rule. The local variables of the external function can be referenced by the internal function, even if the external function has returned.
In fact, the function itself is also an object, and the object has many attributes, among which the closure attribute is related to closure.
closure The attribute defines a tuple containing cell objects, where each cell object in the tuple is used to save the value of the variable in the scope.

func2的id： 139940709362136
res的id：   139940709362136
9
删除函数func1之后： 25
#我们来打印一下res的__closure__属性，可以看出它是由cell对象组成的元组
(<cell>,)
<class> #打印类型结果就是元组
2  #打印cell中的值可以看出，这个cell对象中存储的变量值为2</class></cell>

After triggering the closure, it is possible to reference the upper-level local namespace variable without relying on the local space. Even if the function in which the variable is located is deleted, the closure function can still use it. In fact, The closure function saves it in the __closure__ attribute.

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