Secrets about Python return functions and return values

I recently came into contact with python and saw the concept of returning functions in python. I have only been exposed to function return values ​​ before. For example, python can return int, str, list, dict, etc. Type data, what I want to say here is that python also supports return function.

First look at the basic syntax of python's support for return function

def f():
    print 'call f()...'
    # 定义函数g:
    def g():
        print 'call g()...'
    # 返回函数g:
    return g

Let's implement the sum of variable parameters. Usually, the summation function is defined like this:

def calc_sum(*args):
    ax = 0
    for n in args:
        ax = ax + n  
    return ax

However, what if the sum does not need to be summed immediately, but is calculated as needed in the subsequent code? Instead of returning the summation result, you can return the summation function:

def lazy_sum(*args):
    def sum():
        ax = 0
        for n in args:
            ax = ax + n       
        return ax   
        return sum

When we call lazy_sum(), what is returned is not the summation result, but the summation function:

>>> f = lazy_sum(1, 3, 5, 7, 9)
>>> f
<function lazy_sum.<locals>.sum at 0x101c6ed90>

The result of the sum is actually calculated when function f is called:

>>> f()
25

In this example, we define the function sum in the function lazy_sum, and the internal function sum can refer to the parameters of the external function lazy_sum and local variables. When lazy_sum returns the function sum, the relevant parameters and variables are saved in the returned function. This program structure called "Closure" has great power.

Please note one more thing, when we call lazy_sum(), each call will return a new function, even if the same parameters are passed in:

>>> f1 = lazy_sum(1, 3, 5, 7, 9)
>>> f2 = lazy_sum(1, 3, 5, 7, 9)
>>> f1==f2
False

f1() and f2() The calling results do not affect each other.

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