An article will guide you through Python return functions

1. What is a return function?

Return function, simply put, the return value is a function.

Returns a function, and the result will be returned only when the returned function is called.

2. Closure (implemented in the form of a return function)

Relevant parameters and variables are saved in the returned function, which is called "Closure".

def returnre(a, b):
    def re():
        return a + b


    return re


test01 = returnre(1, 2)
test02 = returnre(1, 2)
print(test01 == test02)  # 每次调用都会返回新生成的函数

#For example, the characteristics of the re function closure in the above example, nested functions defined in the non-global scope , able to remember the enclosing namespace it was in when it was defined.

Let’s look at an example:

def closur(a):
    def myprint():
        print(a)


    return myprint


test = closur(1)
test()

Code analysis:

Variable a is a local variable of the function closure. It should only exist when the function is running. However, because of the characteristics of the closure, the value of a is encapsulated into the return function.

def closuretest02(*args):
    f = []
    for i in args:
        def test():
            return i**2
        f.append(test)
    return f
test = closuretest02(1,2,3,4)
for i in test:
    print(i())

返回闭包时牢记的一点就是：

返回函数不要引用任何循环变量，或者后续会发生变化的变量。

如果一定要引用循环变量怎么办？

方法是再创建一个函数，用该函数的参数绑定循环变量当前的值，无论该循环变量后续如何更改，已绑定到函数参数的值不变：

def count():
    def f(j):
        def g():
            return j*j
        return g
    fs = []
    for i in range(1, 4):
        fs.append(f(i)) # f(i)立刻被执行，因此i的当前值被传入f()
    return fs

再看看结果：

f1, f2, f3 = count()
print(f1())


print(f2())
print(f3())

注：

缺点是代码较长，可利用lambda函数缩短代码。

一个函数可以返回一个计算结果，也可以返回一个函数。返回一个函数时，牢记该函数并未执行，返回函数中不要引用任何可能会变化的变量。

三、拓展

nonlocal

nonlocal适用于嵌套函数中内部函数修改外部变量的值。

def outside():
    a = 1
    print('outside' + str(id(a)))
    def inside():
        nonlocal a
        print('inside ' + str(id(a)))
    inside()
outside()

因为使用nonlocal后，返回了闭包中有父函数的变量，所以父函数那里不会被回收。

四、总结

本文基于Python基础，介绍了返回函数。常见的返回函数的应用。函数作为返回值，闭包在实际应用中需要的点，遇到的难点，提供有效的解决方案。使用Python语言，能够更好的理解。

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