這是Python裝飾器講解的第二部分，上一篇：Python裝飾器Part I：裝飾器簡介
回顧：不帶參數(shù)的裝飾器
在Python裝飾器Part I：裝飾器簡介中，我演示了怎么樣使用無參數(shù)的裝飾器，主要是使用類式裝飾器，因?yàn)檫@樣更容易理解。
如果我們創(chuàng)建了一個(gè)不帶參數(shù)的裝飾器，被裝飾的方法會(huì)傳遞給裝飾器的構(gòu)造器，然后在被裝飾的函數(shù)被調(diào)用的時(shí)候，裝飾器的__call__()方法就會(huì)執(zhí)行。
class decoratorWithoutArguments(object):
def __init__(self, f):
"""
If there are no decorator arguments, the function
to be decorated is passed to the constructor.
"""
print "Inside __init__()"
self.f = f
def __call__(self, *args):
"""
The __call__ method is not called until the
decorated function is called.
"""
print "Inside __call__()"
self.f(*args)
print "After self.f(*args)"
@decoratorWithoutArguments
def sayHello(a1, a2, a3, a4):
print 'sayHello arguments:', a1, a2, a3, a4
print "After decoration"
print "Preparing to call sayHello()"
sayHello("say", "hello", "argument", "list")
print "After first sayHello() call"
sayHello("a", "different", "set of", "arguments")
print "After second sayHello() call"
任何傳遞給被裝飾方法的參數(shù)都將傳遞給__call__()，輸出日志是：
Inside __init__()
After decoration
Preparing to call sayHello()
Inside __call__()
sayHello arguments: say hello argument list
After self.f(*args)
After first sayHello() call
Inside __call__()
sayHello arguments: a different set of arguments
After self.f(*args)
After second sayHello() call
需要注意的是，在“裝飾”階段，只有__init__()會(huì)被調(diào)用；同時(shí)只有在被裝飾方法被調(diào)用的時(shí)候，__call__()才會(huì)被調(diào)用。
帶參數(shù)的裝飾器
現(xiàn)在我們把上面的那個(gè)例子簡單的改動(dòng)一下，看看在添加裝飾器參數(shù)的情況下會(huì)發(fā)生什么情況：
class decoratorWithArguments(object):
def __init__(self, arg1, arg2, arg3):
"""
If there are decorator arguments, the function
to be decorated is not passed to the constructor!
"""
print "Inside __init__()"
self.arg1 = arg1
self.arg2 = arg2
self.arg3 = arg3
def __call__(self, f):
"""
If there are decorator arguments, __call__() is only called
once, as part of the decoration process! You can only give
it a single argument, which is the function object.
"""
print "Inside __call__()"
def wrapped_f(*args):
print "Inside wrapped_f()"
print "Decorator arguments:", self.arg1, self.arg2, self.arg3
f(*args)
print "After f(*args)"
return wrapped_f
@decoratorWithArguments("hello", "world", 42)
def sayHello(a1, a2, a3, a4):
print 'sayHello arguments:', a1, a2, a3, a4
print "After decoration"
print "Preparing to call sayHello()"
sayHello("say", "hello", "argument", "list")
print "after first sayHello() call"
sayHello("a", "different", "set of", "arguments")
print "after second sayHello() call"
從輸出結(jié)果來看，運(yùn)行的效果發(fā)生了明顯的變化：
Inside __init__()
Inside __call__()
After decoration
Preparing to call sayHello()
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: say hello argument list
After f(*args)
after first sayHello() call
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: a different set of arguments
After f(*args)
after second sayHello() call
現(xiàn)在，在“裝飾”階段，構(gòu)造器和__call__()都會(huì)被依次調(diào)用，__call__()也只接受一個(gè)函數(shù)對象類型的參數(shù)，而且必須返回一個(gè)裝飾方法去替換原有的方法，__call__()只會(huì)在“裝飾”階段被調(diào)用一次，接著返回的裝飾方法會(huì)被實(shí)際用在調(diào)用過程中。
盡管這個(gè)行為很合理，構(gòu)造器現(xiàn)在被用來捕捉裝飾器的參數(shù)，而且__call__()不能再被當(dāng)做裝飾方法，相反要利用它來完成裝飾的過程。盡管如此，第一次見到這種與不帶參數(shù)的裝飾器迥然不同的行為還是會(huì)讓人大吃一驚，而且它們的編程范式也有很大的不同。
帶參數(shù)的函數(shù)式裝飾器
最后，讓我們看一下更復(fù)雜的函數(shù)式裝飾器，在這里你不得不一次完成所有的事情：
def decoratorFunctionWithArguments(arg1, arg2, arg3):
def wrap(f):
print "Inside wrap()"
def wrapped_f(*args):
print "Inside wrapped_f()"
print "Decorator arguments:", arg1, arg2, arg3
f(*args)
print "After f(*args)"
return wrapped_f
return wrap
@decoratorFunctionWithArguments("hello", "world", 42)
def sayHello(a1, a2, a3, a4):
print 'sayHello arguments:', a1, a2, a3, a4
print "After decoration"
print "Preparing to call sayHello()"
sayHello("say", "hello", "argument", "list")
print "after first sayHello() call"
sayHello("a", "different", "set of", "arguments")
print "after second sayHello() call"
輸出結(jié)果：
Inside wrap()
After decoration
Preparing to call sayHello()
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: say hello argument list
After f(*args)
after first sayHello() call
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: a different set of arguments
After f(*args)
after second sayHello() call
函數(shù)式裝飾器的返回值必須是一個(gè)函數(shù)，能包裝原有被包裝函數(shù)。也就是說，Python會(huì)在裝飾發(fā)生的時(shí)候拿到并且調(diào)用這個(gè)返回的函數(shù)結(jié)果，然后傳遞給被裝飾的函數(shù)，這就是為什么我們在裝飾器的實(shí)現(xiàn)里嵌套定義了三層的函數(shù)，最里層的那個(gè)函數(shù)是新的替換函數(shù)。
因?yàn)?code>閉包的特性， wrapped_f()在不需要像在類式裝飾器例子中一樣顯示存儲(chǔ)arg1, arg2, arg3這些值的情況下，就能夠訪問這些參數(shù)。不過，這恰巧是我覺得“顯式比隱式更好”的例子。盡管函數(shù)式裝飾器可能更加精簡一點(diǎn)，但類式裝飾器會(huì)更加容易理解并因此更容易被修改和維護(hù)。
