Python Decorators II: Decorator Arguments

October 19, 2008

(本文是(Python3之模式和用法)一书的章节节选第二部分，点击阅读第一部分)

回顾：不含参数的decorators

在前文中，我介绍了如何使用不含参数的decorators，并使用类来实现。因为我发现这样做更容易接受。

如果创建了一个无参decorator，被decorated的函数被传至构造器，每次调用decorated函数时就会调用__call__()方法：

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"

decorated函数的所有参数会被传至__call__()。输出结果是：

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

注意，__init__()是唯一一个被调用执行decoration的方法，每次调用decorated的sayHello()时就会调用__call__()。

含有参数的decorators

现在让我们来修改上面的代码，看看向decorator加入参数后结果是什么。

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"

从输出结果可以看到，加入参数使程序执行发生了很大变化。

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

现在decoration方法调用构造器，然后就马上调用__call__()，后者只能包含一个参数(函数对象)且返回替代原有函数的decorated函数对象。注意当前decoration期间__call__()仅被调用一次，此后从__call__()返回的decorated函数就可以在实际调用中使用了。

虽然这种机制有一定合理性—构造器在这里可获取decorator参数，但__call__()对象不能再作为decorated函数使用了。因此你必须使用__call__()执行decoration—可能第一次遇到这种与无参情况截然不同的方式你会比较吃惊，何况还必须编写和无参decorator完成不同的代码。

含decorator参数的decorator函数

最后，让我们看一个更复杂一点的decorator函数实现，它需要你处理所有细节：

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"

输出结果为：

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

decorator函数的返回值必须是一个封装待decorated函数的函数。也就是说，Python会保存返回函数然后在decoration期间调用，并传递待decorated函数。这也是为何有三层函数的原因：里面那个函数才是被替换的。

由于闭包，wrapped_f()有权访问decorator参数arg1, arg2和arg3，而无需像在class版本中那样显式存储它们。然而，我也是在这里发现了“显胜于隐(explicit is better than implicit)”。即使该函数版本看起来要更加简洁紧凑，但我发现还是类版本容易理解，当然也就容易修改和维护。

下一节内容

在下一节中我会给出decorators的一些实例—基于Python开发的build system—然后在最后一节讨论类decorators。