• 你的第一个装饰器

    你的第一个装饰器

    在上一个例子里，其实我们已经创建了一个装饰器！现在我们修改下上一个装饰器，并编写一个稍微更有用点的程序：

    1. def a_new_decorator(a_func):
    3.     def wrapTheFunction():
    4.         print("I am doing some boring work before executing a_func()")
    6.         a_func()
    8.         print("I am doing some boring work after executing a_func()")
    10.     return wrapTheFunction
    12. def a_function_requiring_decoration():
    13.     print("I am the function which needs some decoration to remove my foul smell")
    15. a_function_requiring_decoration()
    16. #outputs: "I am the function which needs some decoration to remove my foul smell"
    18. a_function_requiring_decoration = a_new_decorator(a_function_requiring_decoration)
    19. #now a_function_requiring_decoration is wrapped by wrapTheFunction()
    21. a_function_requiring_decoration()
    22. #outputs:I am doing some boring work before executing a_func()
    23. #        I am the function which needs some decoration to remove my foul smell
    24. #        I am doing some boring work after executing a_func()

    你看明白了吗？我们刚刚应用了之前学习到的原理。这正是python中装饰器做的事情！它们封装一个函数，并且用这样或者那样的方式来修改它的行为。现在你也许疑惑，我们在代码里并没有使用@符号？那只是一个简短的方式来生成一个被装饰的函数。这里是我们如何使用@来运行之前的代码：

    1. @a_new_decorator
    2. def a_function_requiring_decoration():
    3.     """Hey you! Decorate me!"""
    4.     print("I am the function which needs some decoration to "
    5.           "remove my foul smell")
    7. a_function_requiring_decoration()
    8. #outputs: I am doing some boring work before executing a_func()
    9. #         I am the function which needs some decoration to remove my foul smell
    10. #         I am doing some boring work after executing a_func()
    12. #the @a_new_decorator is just a short way of saying:
    13. a_function_requiring_decoration = a_new_decorator(a_function_requiring_decoration)

    希望你现在对Python装饰器的工作原理有一个基本的理解。如果我们运行如下代码会存在一个问题：

    1. print(a_function_requiring_decoration.__name__)
    2. # Output: wrapTheFunction

    这并不是我们想要的！Ouput输出应该是“a_function_requiring_decoration”。这里的函数被warpTheFunction替代了。它重写了我们函数的名字和注释文档(docstring)。幸运的是Python提供给我们一个简单的函数来解决这个问题，那就是functools.wraps。我们修改上一个例子来使用functools.wraps：

    1. from functools import wraps
    3. def a_new_decorator(a_func):
    4.     @wraps(a_func)
    5.     def wrapTheFunction():
    6.         print("I am doing some boring work before executing a_func()")
    7.         a_func()
    8.         print("I am doing some boring work after executing a_func()")
    9.     return wrapTheFunction
    11. @a_new_decorator
    12. def a_function_requiring_decoration():
    13.     """Hey yo! Decorate me!"""
    14.     print("I am the function which needs some decoration to "
    15.           "remove my foul smell")
    17. print(a_function_requiring_decoration.__name__)
    18. # Output: a_function_requiring_decoration

    现在好多了。我们接下来学习装饰器的一些常用场景。

    蓝本规范:

    1. from functools import wraps
    2. def decorator_name(f):
    3.     @wraps(f)
    4.     def decorated(*args, **kwargs):
    5.         if not can_run:
    6.             return "Function will not run"
    7.         return f(*args, **kwargs)
    8.     return decorated
    10. @decorator_name
    11. def func():
    12.     return("Function is running")
    14. can_run = True
    15. print(func())
    16. # Output: Function is running
    18. can_run = False
    19. print(func())
    20. # Output: Function will not run

    注意：@wraps接受一个函数来进行装饰，并加入了复制函数名称、注释文档、参数列表等等的功能。这可以让我们在装饰器里面访问在装饰之前的函数的属性。