python中抽象方法的委托设计模式

Question

我有以下类用一个额外的DelegatorParent类实现“委托设计模式”：

class DelegatorParent():

    def __init__(self):
        self.a = 'whatever'    

class ConcreteDelegatee():

    def myMethod(self):
        return 'myMethod'


class Delegator(DelegatorParent):

    def __init__(self):
        self.delegatee = ConcreteDelegatee()
        DelegatorParent.__init__(self)

    def __getattr__(self, attrname):
        return getattr(self.delegatee, attrname)

a = Delegator()
result = a.myMethod()

一切看起来都很好。在

现在我想在DelegatorParent中放置一个抽象方法，以确保“myMethod”始终被定义。在

^{pr2}$

你能帮我找到一个“优雅”的方法把“myMethod”从“Delegator”中删除。。。直觉告诉我它在某种程度上是多余的（考虑到定义了一个自定义getattr方法）。在

更重要的是，请注意，在这个实现中，如果我忘记在ConcreteDelegate中定义myMethod，程序将编译，但如果我调用它，它可能会在运行时崩溃委托人.myMethod（），这正是我想通过在DelegatorParent中使用抽象方法来避免的。在

显然，一个简单的解决方案是将@abstractmethod移到Delegator类，但我不想这样做，因为在我的程序中DelegatorParent是一个非常重要的类（Delegator只是一个辅助类）。在

Answer 1

这是我目前的解决方案。它解决了主要的问题（防止具体的delegate忘记定义myMethod），但我还是不太相信，因为我仍然需要在Delegator中定义myMethod，这似乎是多余的

from abc import ABCMeta, abstractmethod

class DelegatorParent(object):
    __metaclass__ = ABCMeta

    def __init__(self):
        self.a = 'whatever'

    @abstractmethod
    def myMethod(self):
        pass


class Delegatee(object):
    def checkExistence(self, attrname):
        if not callable(getattr(self, attrname, None)):
            error_msg = "Can't instantiate " + str(self.__class__.__name__) + " without abstract method " + attrname
            raise NotImplementedError(error_msg)


class ConcreteDelegatee(Delegatee):    
    def myMethod(self):
        return 'myMethod'

    def myMethod2(self):
        return 'myMethod2'


class Delegator(DelegatorParent):
    def __init__(self):
        self.delegatee = ConcreteDelegatee()
        DelegatorParent.__init__(self)
        for method in DelegatorParent.__abstractmethods__:
            self.delegatee.checkExistence(method)

    def myMethod(self, *args, **kw):
        return self.delegatee.myMethod(*args, **kw)

    def __getattr__(self, attrname):
        # Called only for attributes not defined by this class (or its bases).
        # Retrieve attribute from current behavior delegate class instance.
        return getattr(self.delegatee, attrname)



# if I forget to implement myMethod inside ConcreteDelegatee, 
# the following line will correctly raise an exception saying 
# that 'myMethod' is missing inside 'ConcreteDelegatee'.
a = Delegator() 

print a.myMethod() # correctly prints 'myMethod'

print a.myMethod2() #correctly prints 'myMethod2'

Answer 2

您可以决定自动实现委派给ConcreteDelegatee的抽象方法。在

对于每个抽象方法，检查其名称是否存在于ConcreteDelegatee类中，并将此方法作为该类方法的委托实现。在

from abc import ABCMeta, abstractmethod

class DelegatorParent(object):
    __metaclass__ = ABCMeta

    def __init__(self):
        self.a = 'whatever'

    @abstractmethod
    def myMethod(self):
        pass


class Delegatee(object):
    pass


class ConcreteDelegatee(Delegatee):    
    def myMethod(self):
        return 'myMethod'

    def myMethod2(self):
        return 'myMethod2'


class Delegator(DelegatorParent):

    def __new__(cls, *args, **kwargs):
        implemented = set()
        for name in cls.__abstractmethods__:
            if hasattr(ConcreteDelegatee, name):
                def delegated(this, *a, **kw):
                    meth = getattr(this.delegatee, name)
                    return meth(*a, **kw)
                setattr(cls, name, delegated)
                implemented.add(name)
        cls.__abstractmethods__ = frozenset(cls.__abstractmethods__ - implemented)
        obj = super(Delegator, cls).__new__(cls, *args, **kwargs)
        obj.delegatee = ConcreteDelegatee()
        return obj

    def __getattr__(self, attrname):
        # Called only for attributes not defined by this class (or its bases).
        # Retrieve attribute from current behavior delegate class instance.
        return getattr(self.delegatee, attrname)

# All abstract methods are delegared to ConcreteDelegatee
a = Delegator() 

print(a.myMethod()) # correctly prints 'myMethod'

print(a.myMethod2()) #correctly prints 'myMethod2'

这解决了主要问题（防止ConcreteDelegatee忘记定义{}）。如果您忘记了实现其他抽象方法，它们仍然会被检查。在

__new__方法负责委派，它可以释放您的__init__来完成它。在

Answer 3

由于使用ABCMeta，因此必须定义抽象方法。可以从__abstractmethods__集中删除您的方法，但它是frozenset。不管怎样，它包括列出所有抽象方法。在

因此，您可以使用一个简单的描述符来代替__getattr__。在

例如：

class Delegated(object):
    def __init__(self, attrname=None):
        self.attrname = attrname

    def __get__(self, instance, owner):
        if instance is None:
            return self
        delegatee = instance.delegatee
        return getattr(delegatee, self.attrname)


class Delegator(DelegatorParent):
    def __init__(self):
        self.delegatee = ConcreteDelegatee()
        DelegatorParent.__init__(self)

    myMethod = Delegated('myMethod')

这里的一个优点是：开发人员拥有“myMethod”被委托的明确信息。在

如果您尝试：

^{pr2}$

它起作用了！但是，如果您忘记在Delegator类中实现myMethod，则会出现典型错误：

Traceback (most recent call last):
  File "script.py", line 40, in <module>
    a = Delegator()
TypeError: Can't instantiate abstract class Delegator with abstract methods myMethod

编辑

这种实现可以概括如下：

class DelegatorParent():
    __metaclass__ = ABCMeta

    @abstractmethod
    def myMethod1(self):
        pass

    @abstractmethod
    def myMethod2(self):
        pass

    def __init__(self):
        self.a = 'whatever'


class ConcreteDelegatee1():
    def myMethod1(self):
        return 'myMethod1'


class ConcreteDelegatee2():
    def myMethod2(self):
        return 'myMethod2'


class DelegatedTo(object):
    def __init__(self, attrname):
        self.delegatee_name, self.attrname = attrname.split('.')

    def __get__(self, instance, owner):
        if instance is None:
            return self
        delegatee = getattr(instance, self.delegatee_name)
        return getattr(delegatee, self.attrname)


class Delegator(DelegatorParent):
    def __init__(self):
        self.delegatee1 = ConcreteDelegatee1()
        self.delegatee2 = ConcreteDelegatee2()
        DelegatorParent.__init__(self)

    myMethod1 = DelegatedTo('delegatee1.myMethod1')
    myMethod2 = DelegatedTo('delegatee2.myMethod2')


a = Delegator()
result = a.myMethod2()

在这里，我们可以指定delegatee名称和delegatee方法。在