python - 完美模仿继承与组合

Question

我试图以这样的方式包装第三方包中的类，使我的新类看起来与第三方类的子类完全相同。第三方类不支持继承，并且具有重要的功能，例如具有 __getitem__ 方法的函数。我可以使用基于 Wrapping a class whose methods return instances of that class 和 How can I intercept calls to python's "magic" methods in new style classes? 的解决方案包装几乎每个属性和方法。不过，我还是需要重写第三方类的__init__方法。我怎样才能做到这一点？注意:我正在使用新式类。

到目前为止的代码:

import copy

class WrapperMetaclass(type):
    """
    Works with the `Wrapper` class to create proxies for the wrapped object's magic methods.
    """
    def __init__(cls, name, bases, dct):

        def make_proxy(name):
            def proxy(self, *args):
                return getattr(self._obj, name)
            return proxy

        type.__init__(cls, name, bases, dct)
        if cls.__wraps__:
            ignore = set("__%s__" % n for n in cls.__ignore__.split())
            for name in dir(cls.__wraps__):
                if name.startswith("__"):
                    if name not in ignore and name not in dct:
                        setattr(cls, name, property(make_proxy(name)))

class Wrapper(object):
    """
    Used to provide a (nearly) seamless inheritance-like interface for classes that do not support direct inheritance.
    """

    __metaclass__ = WrapperMetaclass
    __wraps__  = None
    # note that the __init__ method will be ignored by WrapperMetaclass
    __ignore__ = "class mro new init setattr getattr getattribute dict"

    def __init__(self, obj):
        if self.__wraps__ is None:
            raise TypeError("base class Wrapper may not be instantiated")
        elif isinstance(obj, self.__wraps__):
            self._obj = obj
        else:
            raise ValueError("wrapped object must be of %s" % self.__wraps__)

    def __getattr__(self, name):
        if name is '_obj':
            zot = 1
        orig_attr = self._obj.__getattribute__(name)
        if callable(orig_attr) and not hasattr(orig_attr, '__getitem__'):
            def hooked(*args, **kwargs):
                result = orig_attr(*args, **kwargs)
                if result is self._obj:
                    return self
                elif isinstance(result, self.__wraps__):
                    return self.__class__(result)
                else:
                    return result
            return hooked
        else:
            return orig_attr

    def __setattr__(self, attr, val):
        object.__setattr__(self, attr, val)
        if getattr(self._obj, attr, self._obj) is not self._obj: # update _obj's member if it exists
            setattr(self._obj, attr, getattr(self, attr))

class ClassToWrap(object):
    def __init__(self, data):
        self.data = data

    def theirfun(self):
        new_obj = copy.deepcopy(self)
        new_obj.data += 1
        return new_obj

    def __str__(self):
        return str(self.data)

class Wrapped(Wrapper):
    __wraps__ = ClassToWrap

    def myfun(self):
        new_obj = copy.deepcopy(self)
        new_obj.data += 1
        return new_obj

# can't instantiate Wrapped directly! This is the problem!
obj = ClassToWrap(0)
wr0 = Wrapped(obj)
print wr0
>> 0
print wr0.theirfun()
>> 1

这可行，但为了真正无缝的类似继承的行为，我需要直接实例化 Wrapped，例如

wr0 = Wrapped(0)

当前抛出

ValueError: wrapped object must be of <class '__main__.ClassToWrap'>

我尝试通过在 __init__ 中定义 WrapperMetaclass 的新代理来进行覆盖，但很快遇到了无限递归。

我的代码库很复杂，用户的技能水平各不相同，因此我无法使用猴子修补或修改示例类 ClassToWrap 或 Wrapped 定义的解决方案。我真的希望对上面的代码进行扩展来覆盖 Wrapped.__init__ 。

请注意，这个问题不仅仅是例如的重复。 Can I exactly mimic inheritance behavior with delegation by composition in Python? 。该帖子没有提供任何与我已在此处提供的内容一样详细的答案。

Answer 1

听起来你只是想要 Wrapper.__init__方法与目前的工作方式不同。而不是采用 __wraps__ 的现有实例类，它应该采用另一个类在其构造函数中期望的参数并为您构建实例。尝试这样的事情:

def __init__(self, *args, **kwargs):
    if self.__wraps__ is None:
        raise TypeError("base class Wrapper may not be instantiated")
    else:
        self._obj = self.__wraps__(*args, **kwargs)

如果你想要Wrapper为了出于某种原因保持不变，您可以将逻辑放入新的 Wrapped.__init__ 中方法替代:

def __init__(self, data): # I'm explicitly naming the argument here, but you could use *args
    super(self, Wrapped).__init__(self.__wraps__(data)) # and **kwargs to make it extensible