python 装饰器

1. 装饰器定义

装饰器本质上是一个Python函数，它可以让其他函数在不需要做任何代码变动的前提下增加额外功能，装饰器的返回值也是一个函数对象。它经常用于有切面需求的场景，比如：插入日志、性能测试、事务处理、缓存、权限校验等场景。装饰器是解决这类问题的绝佳设计，有了装饰器，我们就可以抽离出大量与函数功能本身无关的雷同代码并继续重用。

2. 回顾函数（学习装饰器的前提条件）

2.1 一切皆对象——函数

def hi(name="yasoob"):
    return "hi " + name
 
print(hi())
# output: 'hi yasoob'
 
# 我们甚至可以将一个函数赋值给一个变量，比如
greet = hi
# 我们这里没有在使用小括号，因为我们并不是在调用hi函数
# 而是在将它放在greet变量里头。我们尝试运行下这个
 
print(greet())
# output: 'hi yasoob'
 
# 如果我们删掉旧的hi函数，看看会发生什么！
del hi
print(hi())
#outputs: NameError
 
print(greet())
#outputs: 'hi yasoob'

2.2 函数中定义函数（嵌套函数）

def hi(name="yasoob"):
    print("now you are inside the hi() function")
 
    def greet():
        return "now you are in the greet() function"
 
    def welcome():
        return "now you are in the welcome() function"
 
    print(greet())
    print(welcome())
    print("now you are back in the hi() function")
 
hi()
#output:now you are inside the hi() function
#       now you are in the greet() function
#       now you are in the welcome() function
#       now you are back in the hi() function
 
# 上面展示了无论何时你调用hi(), greet()和welcome()将会同时被调用。
# 然后greet()和welcome()函数在hi()函数之外是不能访问的，比如：
 
greet()
#outputs: NameError: name 'greet' is not defined

2.3 从函数中返回函数

def hi(name="yasoob"):
    def greet():
        return "now you are in the greet() function"
 
    def welcome():
        return "now you are in the welcome() function"
 
    if name == "yasoob":
        return greet
    else:
        return welcome
 
a = hi()
print(a)
#outputs: <function greet at 0x7f2143c01500>
 
#上面清晰地展示了`a`现在指向到hi()函数中的greet()函数
#现在试试这个
 
print(a())
#outputs: now you are in the greet() function
hi()()
#outputs: now you are in the greet() function

在 if/else 语句中我们返回 greet 和 welcome，而不是 greet() 和 welcome()。为什么那样？这是因为当你把一对小括号放在后面，这个函数就会执行；然而如果你不放括号在它后面，那它可以被到处传递，并且可以赋值给别的变量而不去执行它。
当我们写下 a = hi()，hi() 会被执行，而由于 name 参数默认是 yasoob，所以函数 greet 被返回了。如果我们把语句改为 a = hi(name = “ali”)，那么 welcome 函数将被返回。我们还可以打印出 hi()()，这会输出 now you are in the greet() function。

2.4 将函数作为参数传给另一个函数

def hi():
    return "hi yasoob!"
 
def doSomethingBeforeHi(func):
    print("I am doing some boring work before executing hi()")
    print(func())
 
doSomethingBeforeHi(hi)
#outputs:I am doing some boring work before executing hi()
#        hi yasoob!

3. 第一个装饰器

（过去写法）即原型，Python Version < 2.4，2004年以前

def a_new_decorator(a_func):
 
    def wrapTheFunction():
        print("I am doing some boring work before executing a_func()")
 
        a_func()
 
        print("I am doing some boring work after executing a_func()")
 
    return wrapTheFunction
 
def a_function_requiring_decoration():
    print("I am the function which needs some decoration to remove my foul smell")
 
a_function_requiring_decoration()
#outputs: "I am the function which needs some decoration to remove my foul smell"
 
a_function_requiring_decoration = a_new_decorator(a_function_requiring_decoration)
#now a_function_requiring_decoration is wrapped by wrapTheFunction()
 
a_function_requiring_decoration()
#outputs:I am doing some boring work before executing a_func()
#        I am the function which needs some decoration to remove my foul smell
#        I am doing some boring work after executing a_func()

（现在写法）后面版本的Python中支持了@语法糖

def a_new_decorator(a_func):
 
    def wrapTheFunction():
        print("I am doing some boring work before executing a_func()")
 
        a_func()
 
        print("I am doing some boring work after executing a_func()")
 
    return wrapTheFunction
 
 @a_new_decorator
def a_function_requiring_decoration():
    print("I am the function which needs some decoration to remove my foul smell")
 
a_function_requiring_decoration()
#outputs:I am doing some boring work before executing a_func()
#        I am the function which needs some decoration to remove my foul smell
#        I am doing some boring work after executing a_func()

运行如下代码会存在一个问题：

print(a_function_requiring_decoration.__name__)
# 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：

from functools import wraps
 
def a_new_decorator(a_func):
    @wraps(a_func)
    def wrapTheFunction():
        print("I am doing some boring work before executing a_func()")
        a_func()
        print("I am doing some boring work after executing a_func()")
    return wrapTheFunction
 
@a_new_decorator
def a_function_requiring_decoration():
    """Hey yo! Decorate me!"""
    print("I am the function which needs some decoration to "
          "remove my foul smell")
 
print(a_function_requiring_decoration.__name__)
# Output: a_function_requiring_decoration

4. 高阶装饰器

4.1 判断是否调用装饰器

from functools import wraps
def decorator_name(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        if not can_run:
            return "Function will not run"
        return f(*args, **kwargs)
    return decorated
 
@decorator_name
def func():
    return("Function is running")
 
can_run = True
print(func())
# Output: Function is running
 
can_run = False
print(func())
# Output: Function will not run

4.2 带参数的装饰器

打出log信息，而且还需指定log的级别

def logging(level):
    def wrapper(func):
        def inner_wrapper(*args, **kwargs):
            print "[{level}]: enter function {func}()".format(
                level=level,
                func=func.__name__)
            return func(*args, **kwargs)
        return inner_wrapper
    return wrapper

@logging(level='INFO')
def say(something):
    print "say {}!".format(something)

# 如果没有使用@语法，等同于
# say = logging(level='INFO')(say)

@logging(level='DEBUG')
def do(something):
    print "do {}...".format(something)

if __name__ == '__main__':
    say('hello')
    do("my work")

参考：

• 详解 python 装饰器函数
• Python 函数装饰器
• geeksforgeeks Decorators in Python
• realpython.com Primer on Python Decorators
• programiz Python Decorators