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decorator
中func()
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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