python之装饰器

作者: howie6879 | 来源:发表于2016-07-31 19:51 被阅读604次

    1.认识装饰器

    在python中,对于一个函数,若想在其运行前后做点什么,那么装饰器是再好不过的选择,话不多说,上代码。

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 01.py
    __author__ = 'howie'
    from functools import wraps
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            print("%s was called" % func.__name__)
            func(*args, **kwargs)
        return wrapper
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    
    outputs:
    hello was called
    Hello howie!
    

    这段代码,初看之下,确实不是很理解,接下来一步一步分析,看看装饰器到底是怎么工作的。

    2.装饰器原理

    在python中,方法允许作为参数传递,想在某个函数执行前后加点料,也可以这样简单实现。

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-1.py
    __author__ = 'howie'
    def decorator(func):
        print("%s was called" % func.__name__)
        func()
    def hello(name="howie"):
        print("Hello %s!" % name)
    decorator(hello)
    

    由此,上面代码也可以这样写:

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-2.py
    __author__ = 'howie'
    def decorator(func):
        print("%s was called" % func.__name__)
        func()
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello
    

    两段代码执行后:

    outputs:
    hello was called
    Hello howie!
    

    表面上看来,02-2.py代码看起来也可以很好地执行啊,可请注意,在末尾处,hello只是函数名称,它并不能被调用,若执行hello(),就会报TypeError: 'NoneType' object is not callable对象不能调用错误,这是自然,在decoratorfunc()直接将传入的函数实例化了,有人会想,那如果这样改呢?

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-3.py
    __author__ = 'howie'
    def decorator(func):
        print("%s was called" % func.__name__)
        return func
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    

    确实,这样改是可以,可有没有想过,若想在函数执行结束后加点装饰呢?这样便行不通了,可能又有人会想,若这样改呢?

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-4.py
    __author__ = 'howie'
    def decorator(func):
        print("%s was called" % func.__name__)
        func()
        return bye
    def bye():
        print("bye~")
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    

    这样写看起来,恩,怎么说呢,总有种没有意义的感觉,不如直接将在外部的函数放进decorator中,如下:

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-5.py
    __author__ = 'howie'
    def decorator(func):
        def wrapper():
          print("%s was called" % func.__name__)
          func()
          print("bye~")
        return wrapper
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    

    执行:

    outputs:
    hello was called
    Hello howie!
    bye~
    

    怎么样,输出的结果是不是符合要求,其实简单来看的话,可以这样理解hello()==decorator(hello)()==wrapper(),最后其实就是执行wrapper()函数而已,事实就是如此的简单,不妨来验证一下:

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-6.py
    __author__ = 'howie'
    def decorator(func):
        def wrapper():
          print("%s was called" % func.__name__)
          func()
          print("bye~")
        return wrapper
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    print(hello.__name__)
    
    outputs:
    hello was called
    Hello howie!
    bye~
    wrapper
    

    果然就是执行了wrapper函数,解决问题的同时也会出现新的问题,那便是代码中本来定义的hello函数岂不是被wrapper函数覆盖了,又该如何解决这个问题呢?这时候functions.wraps就可以登场了,代码如下:

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 02-7.py
    __author__ = 'howie'
    from functools import wraps
    def decorator(func):
        @wraps(func)
        def wrapper():
          print("%s was called" % func.__name__)
          func()
          print("bye~")
        return wrapper
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello()
    print(hello.__name__)
    

    执行代码:

    outputs:
    hello was called
    Hello howie!
    bye~
    hello
    

    functions.wraps作用是不是一目了然哈到了这一步,再看01.py的代码,是不是代码结构清晰明了,只不过多了个参数

    #!/usr/bin/env
    # -*-coding:utf-8-*-
    # script: 01.py
    __author__ = 'howie'
    from functools import wraps
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            print("%s was called" % func.__name__)
            func(*args, **kwargs)
        return wrapper
    @decorator
    def hello(name="howie"):
        print("Hello %s!" % name)
    hello('world')
    

    猜都猜得到执行后输出什么了。

    3.结语

    只要了解装饰器原理,不管是带参数的装饰器,还是装饰器类,都是小菜一碟。
    若有错误,尽请指出。

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