Python Decorators II: Decorator Arguments

October 19, 2008

(本文是(Python3之模式和用法)一書的章節節選第二部分，點擊閱讀第一部分)

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，并使用類來實現。因為我發現這樣做更容易接受。

如果創建了一個無參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加入參數后結果是什么。

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"

從輸出結果可以看到，加入參數使程序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，然后就馬上調用__call__()，后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，此后從__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—構造器在這里可獲取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

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

def decoratorFunctionWithArguments(arg1, arg2, arg3):

def wrap(f):

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"

輸出結果為：

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函數的返回值必須是一個封裝待decorated函數的函數。也就是說，Python會保存返回函數然后在decoration期間調用，并傳遞待decorated函數。這也是為何有三層函數的原因：里面那個函數才是被替換的。

由于閉包，wrapped_f()有權訪問decorator參數arg1, arg2和arg3，而無需像在class版本中那樣顯式存儲它們。然而，我也是在這里發現了“顯勝于隱(explicit is better than implicit)”。即使該函數版本看起來要更加簡潔緊湊，但我發現還是類版本容易理解，當然也就容易修改和維護。

下一節內容

在下一節中我會給出decorators的一些實例—基于Python開發的build system—然后在最后一節討論類decorators。

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