Python notes (17): closure

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Closure

The concept of closure in function , Simply put, a function definition refers to variables defined outside the function , And the function can be executed outside its definition environment . Such a function is called closure . In fact, closure can be seen as a more generalized concept of function . Because it is no longer a function defined in the traditional sense .

The concept of closure is not limited to Python in , In almost any programming language .

The condition of closure ：

1. The internal function is defined in the external function
2. An external function has a return value
3. The return value is ： Internal function
4. The inner function also references the variables of the outer function

The format is as follows ：

def  External function ():
    ...
    def  Internal function ():
        ...
    return  Internal function 
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Sample code

def func():
    a = 100

    def inner_func():
        b = 200
        print(a, b)

    return inner_func


x = func()
print(x)  # <function func.<locals>.inner_func at 0x0000021704CD9620>
x()  # 100 200
#  So you can call it directly inner_func function , If return If you don't return an internal function, there will be no output 
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Using closures can also accomplish the effect of counters

def generate_count():
    container = [0]

    def add_one():
        container[0] += 1
        print(f" This is the first {container[0]} Secondary call ")

    return add_one


count = generate_count()
count()  #  This is the first 1 Secondary call 
count()  #  This is the first 2 Secondary call 
count()  #  This is the first 3 Secondary call 
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Closure shortcoming As shown below ：

1. The scope is not so intuitive
2. Because variables will not be garbage collected, there is a certain memory occupation problem .

Closure effect As shown below ：

1. You can use peer scopes
2. Read internal variables of other elements
3. Extend scope