从装饰类继承

Posted 2023-02-23

【中文标题】从装饰类继承

【英文标题】：Inheriting from decorated classes

【发布时间】：2011-11-20 14:18:14

【问题描述】：

我正在尝试用另一个班级装饰一个班级。我也想，但我得到了一些错误。这是我的代码：

class Decorator:
    def __init__(self, decorated):
        pass

@Decorator
class Foo:
    pass

class Goo(Foo):
    pass

当我尝试从Foo 子类化时得到的错误是：

回溯（最近一次通话最后一次）： 文件“test.py”，第 9 行，在 类咕（Foo）： TypeError: __init__() 正好采用 2 个位置参数（给定 4 个）

通过向Decorator添加另一个初始化函数...

def __init__(self, *args):
    for arg in args:
        print(arg)

...我得到以下输出：

咕 (<__main__.decorator object at>,) '__module__': '__main__'

这些参数是什么？我应该如何在Decorator 中使用它们？

【参考方案1】：

我将尝试回答“这些参数是什么”的问题。这段代码：

@Decorator
class Foo:
    pass

相当于：

class Foo:
    pass
Foo = Decorator(Foo)

这意味着Foo 最终成为Decorator 类的实例，而不是一个类。

当您尝试将此实例用作类的基类 (Goo) 时，Python 必须确定一个元类，用于创建新类。在这种情况下，它将使用等于Decorator 的Foo.__class__。然后它将使用(name, bases, dict) 参数调用元类并期望它返回一个新类。

这就是你在Decorator.__init__ 中最终得到这些参数的方式。

可以在此处找到有关此内容的更多信息： http://www.python.org/download/releases/2.2.3/descrintro/#metaclasses （特别是“当执行类语句时......”部分）

【参考方案2】：

您是否尝试在定义类之后向该类添加 MixIn？ 如果是这样，您可以通过这种方式注入 MixIn：

def inject_class(mixin):
    def _inject_class(cls):
        return type(cls.__name__,(mixin,)+cls.__bases__,dict(cls.__dict__))
    return _inject_class

class MixIn(object):
    def mix(self):
        print('mix')

@inject_class(MixIn)
class Foo(object):
    def foo(self):
        print('foo')

class Goo(Foo):
    def goo(self):
        print('goo')

goo=Goo()
goo.mix()
goo.foo()
goo.goo()

打印

mix
foo
goo

---

如果你不想要inject_class 的通用性，你可以制作一个专门的类装饰器，它只混入Decorator：

def decorate(cls):
    class Decorator(object):
        def deco(self):
            print('deco')
    return type(cls.__name__,(Decorator,)+cls.__bases__,dict(cls.__dict__))

@decorate
class Foo(object):
    def foo(self):
    print('foo')

结果是一样的。

【参考方案3】：

我遇到了同样的问题，以下解决方案对我有用：

from functools import update_wrapper
class decoratorBase():
    def __new__(cls, logic):
        self = object.__new__(cls)
        self.__init__(logic)
        def new (cls):
            #cls is the decorated class type, not the decorator class type itself
            self._createInstance(cls)
            self._postInstanceCreation()
            return self
        self._logic.__new__ = new
        #return the wrapped class and not a wrapper
        return self._logic
    def __init__(self, logic):
        #logic is the decorated class
        self._logic = logic
    def _createInstance(self, cls):
        self._logicInstance = object.__new__(cls)
        self._logicInstance.__init__()
    def _postInstanceCreation(self):
        pass

class factory(decoratorBase):
    def __init__(self, *largs, **kwargs):
        super().__init__(*largs, **kwargs)
        self.__instance = None
    def _createInstance(self, cls):
        self._logicInstance = None
        self._cls = cls
    def _postInstanceCreation(self):
        update_wrapper(self, self._cls)
    def __call__(self, userData, *largs, **kwargs):
        logicInstance = object.__new__(self._cls)
        logicInstance.__init__(*largs, **kwargs)
        logicInstance._update(userData)
        return logicInstance

class singelton(decoratorBase):
    def _postInstanceCreation(self):
        update_wrapper(self, self._logicInstance)
    def __call__(self, userData):
        self._logicInstance._update(userData)
        return self._logicInstance

class base():
    def __init__(self):
        self.var = 0
        print ("Create new object")
    def __call__(self):
        self.var += self._updateValue()
    def _update(self, userData):
        print ("Update object static value with 0".format(userData))
        self.var = userData

@factory
class factoryTestBase(base):

    def __call__(self):
        super().__call__()
        print("I'm a factory, here is the proof: 0".format(self.var))
    def _updateValue(self):
        return 1

class factoryTestDerived(factoryTestBase):
    def _updateValue(self):
        return 5

@singelton
class singeltonTestBase(base):
    def __call__(self):
        super().__call__()
        print("I'm a singelton, here is the proof: 0".format(self.var))
    def _updateValue(self):
        return 1

class singeltonTestDerived(singeltonTestBase):
    def _updateValue(self):
        return 5

这种方法的神奇之处在于 __new__() 方法的重载，对于装饰器本身以及装饰器返回的“包装器”也是如此。我将单词包装器放在引号中，因为实际上没有包装器。相反，装饰类由装饰器交替并返回。使用此方案，您可以从装饰类继承。最重要的是修饰类的__new__()方法的变化，由以下几行组成：

        def new (cls):
            self._createInstance(cls)
            self._postInstanceCreation()
            return self
        self._logic.__new__ = new

使用它，您可以在从装饰类创建对象期间访问装饰器方法，例如self._createInstance()。您甚至有机会从您的装饰器继承（如示例中所示）。

现在让我们运行一个简单的例子：

>>> factoryObjCreater = factoryTestBase()
>>> factoryObj1 = factoryObjCreater(userData = 1)
Create new object
Update object static value with 1
>>> factoryObj2 = factoryObjCreater(userData = 1)
Create new object
Update object static value with 1
>>> factoryObj1()
I'm a factory, here is the proof: 2
>>> factoryObj2()
I'm a factory, here is the proof: 2
>>> factoryObjDerivedCreater = factoryTestDerived()
>>> factoryObjDerived1 = factoryObjDerivedCreater(userData = 2)
Create new object
Update object static value with 2
>>> factoryObjDerived2 = factoryObjDerivedCreater(userData = 2)
Create new object
Update object static value with 2
>>> factoryObjDerived1()
I'm a factory, here is the proof: 7
>>> factoryObjDerived2()
I'm a factory, here is the proof: 7
>>> singeltonObjCreater = singeltonTestBase()
Create new object
>>> singeltonObj1 = singeltonObjCreater(userData = 1)
Update object static value with 1
>>> singeltonObj2 = singeltonObjCreater(userData = 1)
Update object static value with 1
>>> singeltonObj1()
I'm a singelton, here is the proof: 2
>>> singeltonObj2()
I'm a singelton, here is the proof: 3
>>> singeltonObjDerivedCreater = singeltonTestDerived()
Create new object
>>> singeltonObjDerived1 = singeltonObjDerivedCreater(userData = 2)
Update object static value with 2
>>> singeltonObjDerived2 = singeltonObjDerivedCreater(userData = 2)
Update object static value with 2
>>> singeltonObjDerived1()
I'm a singelton, here is the proof: 7
>>> singeltonObjDerived2()
I'm a singelton, here is the proof: 12
>>>