解释Python装饰器的工作方式 [英] Explain how Python decorator works

问题描述

这是python装饰器的示例。我无法理解它的工作方式。请向我解释给定示例的控制流程。我将非常有义务。

This is an example for python decorator. I am unable to understand the way it works. Please explain me the given example's control flow. I will be highly obliged.

def helloSolarSystem(original_function):
   def new_function(*args, **kwargs):
        original_function(*args, **kwargs)  
        print("Hello, solar system!")
   return new_function

def helloGalaxy(original_function):
    def new_function(*args, **kwargs):
        original_function(*args, **kwargs)  
        print("Hello, galaxy!")
    return new_function

@helloGalaxy
@helloSolarSystem
def hello(targetName=None):
     if targetName:
        print("Hello, " +  targetName +"!")
     else:
        print("Hello, world!")
hello("Earth")

推荐答案

装饰器是应用高级功能的语法糖。在Python中。高阶函数是将一个或多个函数作为输入并返回一个函数的函数。即

Decorators are syntactic sugar for applying higher-order functions in Python. A higher-order function is a function that takes one or more functions as inputs and returns a function. i.e.

h(x) = f(g(x))

其中， f（）是一个高阶函数，它使用单个参数的函数 g（x），并返回单个参数 h（x）的函数。您可以将 f（）视为修改 g（）的行为。

where here f() is a higher-order function that takes a function of a single argument, g(x), and returns a function of a single argument, h(x). You can think of f() as modifying the behaviour of g().

高阶函数是可组合（按定义），因此在您的特定示例，装饰器语法，

Higher-order functions are composable (by definition), so in your specific example, the decorator syntax,

@helloGalaxy
@helloSolarSystem
def hello(targetName=None):
    ...

等于，

hello = helloGalaxy(helloSolarSystem(hello))

通过将 hello 替换为 helloSolarSystem ，然后将其结果替换为 helloGalaxy ，我们得到等效的函数调用，

By substituting hello into helloSolarSystem, then the result of that into helloGalaxy, we get the equivalent function call,

def hello(targetName=None):
    if targetName:                            |        
        print("Hello, " + targetName + "!")   |  (1)  |
    else:                                     |       |  (2)   |
        print("Hello, world!")                |       |        |  (3)
    print("Hello, solar system!")                     |        |
    print("Hello, galaxy!")                                    |

其中（1）是原始 hello（）的应用，（2）是应用程序，

where (1) is the application of the original hello(), (2) is the application of,

def helloSolarSystem(original_function):
    def new_function(*args, **kwargs):
        original_function(*args, **kwargs)   <-- (1)
        print("Hello, solar system!")
    return new_function

并且（3）是以下应用，

and (3) is the application of,

def helloGalaxy(original_function):
    def new_function(*args, **kwargs):
        original_function(*args, **kwargs)   <-- (2)
        print("Hello, galaxy!")
    return new_function

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