Python Decorators（二）：Decorator參數__Python

Python Decorators II: Decorator Arguments

October 19, 2008

 

（本文是Python 3 Patterns & Idioms（Python3之模式和用法）一書的章節節選第二部分，點擊這裡閱讀第一部分）

 

回顧：不含參數的decorators

在前文中，我介紹了如何使用不含參數的decorators，並使用類來實現。因為我發現這樣做更容易接受。

如果建立了一個無參decorator，被decorated的函數被傳至構造器，每次調用decorated函數時就會調用__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函數的所有參數會被傳至__call__()。輸出結果是：

 

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__()是唯一一個被調用執行decoration的方法，每次調用decorated的sayHello()時就會調用__call__()。

 

含有參數的decorators

現在讓我們來修改上面的代碼，看看向decorator加入參數後結果是什麼。

 

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"

 

從輸出結果可以看到，加入參數使程式執行發生了很大變化。

 

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

 

現在decoration方法調用構造器，然後就馬上調用__call__()，後者只能包含一個參數（函數對象）且返回替代原有函數的decorated函數對象。注意當前decoration期間__call__()僅被調用一次，此後從__call__()返回的decorated函數就可以在實際調用中使用了。

雖然這種機制有一定合理性—構造器在這裡可擷取decorator參數，但__call__()對象不能再作為decorated函數使用了。因此你必須使用__call__()執行decoration—可能第一次遇到這種與無參情況截然不同的方式你會比較吃驚，何況還必須編寫和無參decorator完成不同的代碼。

 

含decorator參數的decorator函數

最後，讓我們看一個更複雜一點的decorator函數實現，它需要你處理所有細節：

 

def decoratorFunctionWithArguments(arg1, arg2, arg3):

    def wrap(f):