如何在派生的python类上操作装饰器？

Question

我想使用装饰器对派生类做一些事情（例如注册类或其他东西）。这是我的代码：

from functools import wraps

class Basic(object):
    def __init__(self):
        print "Basic::init"

def myDeco(name):
    # define the decorator function that acts on the actual class definition
    def __decorator(myclass):

        # do something here with the information
        print name, myclass

        # do the wrapping of the class 
        @wraps(myclass)
        def __wrapper(*args, **kwargs):
            return myclass( *args, **kwargs)

        # return the actual wrapper here
        return __wrapper

    # return the decorator to act on the class definition
    return __decorator

@myDeco("test")
class Derived(Basic):
    def __init__(self):
        super(Derived, self).__init__()
        print "Derived::init"


instance = Derived()

这给出了以下错误：

TypeError: must be type, not function

当Derived 中的super 方法被调用时。我假设变量Derived 不再是type，而是函数__decorator 实际上。

我需要如何更改装饰器（并且只有装饰器）才能解决此问题？

Answer 1

您正在用函数替换装饰类，因此类定义失败。

具体来说，super(Derived, self).__init__() 现在将一个函数传递给super()：

>>> super(Derived, object())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be type, not function
>>> type(Derived)
<type 'function'>

类装饰器通常会更改类，添加、删除或更改其属性，然后再次返回类对象。

您应该不将Derived 替换为新类，因为您的Derived.__init__() 按名称引用自身。用另一个类替换该名称会导致痛苦（例如无限递归）。

确实工作的示例类装饰器：

def my_deco(name):
    # define the decorator function that acts on the actual class definition
    def decorator(cls):

        # do something here with the information
        print name, cls

        setattr(class, name, 'Hello world!')

        # return the original, augmented class
        return cls

    # return the decorator to act on the class definition
    return decorator

Answer 2

您的程序中有两个错误：

from functools import wraps

class Basic(object):
    def __init__(self):
        print "Basic::init"

def myDeco(name):
    # define the decorator function that acts on the actual class definition
    def __decorator(myclass):

        # do something here with the information
        print name, myclass

        # do the wrapping of the class
        class Wrap(myclass): # error 1
            realSuper = myclass # I dislike this. has anyone an other aproach
            def __new__(*args, **kwargs):
                return myclass.__new__(*args, **kwargs)

        # return the actual wrapper here
        return Wrap

    # return the decorator to act on the class definition
    return __decorator

@myDeco("test")
class Derived(Basic):
    def __init__(self):
        super(Derived.realSuper, self).__init__() # error 2
        print "Derived::init"


instance = Derived()

现在给出以下输出：

test <class '__main__.Derived'>
Basic::init
Derived::init

首先你需要一个其他人指出的类。

第二个super(Derived, self).__init__()造成了无限递归：

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    exec s
  File "<string>", line 32, in <module>
  File "<string>", line 28, in __init__
  File "<string>", line 28, in __init__
  File "<string>", line 28, in __init__
  File "<string>", line 28, in __init__

原因参见下面的讨论。