1、问题背景
在Python中，super方法用于在子类中调用父类的方法。Guido van Rossum曾给出了一个纯Python实现的super方法，以便更好地理解其工作原理。然而，在这个实现中，存在一个问题：当传入的对象不是要调用的父类的实例时，该实现会出错。

2、解决方案
为了解决这个问题，需要对Guido的实现进行修改。具体来说，需要在__getattr__()方法中添加一个检查，以确保传入的对象是给定类型的实例。如果传入的对象不是给定类型的实例，那么就引发一个错误。

以下是修改后的__getattr__()方法：

    def __getattr__(self, attr):if isinstance(self.__obj__, self.__type__):starttype = self.__obj__.__class__else:# 检查传入的对象是否是给定类型的实例if not isinstance(self.__obj__, self.__type__):raise TypeError("obj must be an instance or subtype of type")starttype = self.__obj__mro = iter(starttype.__mro__)for cls in mro:if cls is self.__type__:break# Note: mro is an iterator, so the second loop# picks up where the first one left off!for cls in mro:if attr in cls.__dict__:x = cls.__dict__[attr]if hasattr(x, "__get__"):x = x.__get__(self.__obj__)return xraise AttributeError, attr

通过这个修改，就可以确保传入的对象是给定类型的实例，从而避免了错误的发生。

以下是修改后的代码示例：

class Super(object):def __init__(self, type, obj=None):self.__type__ = typeself.__obj__ = objdef __get__(self, obj, type=None):if self.__obj__ is None and obj is not None:return Super(self.__type__, obj)else:return selfdef __getattr__(self, attr):# 检查传入的对象是否是给定类型的实例if not isinstance(self.__obj__, self.__type__):raise TypeError("obj must be an instance or subtype of type")if isinstance(self.__obj__, self.__type__):starttype = self.__obj__.__class__else:starttype = self.__obj__mro = iter(starttype.__mro__)for cls in mro:if cls is self.__type__:break# Note: mro is an iterator, so the second loop# picks up where the first one left off!for cls in mro:if attr in cls.__dict__:x = cls.__dict__[attr]if hasattr(x, "__get__"):x = x.__get__(self.__obj__)return xraise AttributeError, attrclass A(object):def m(self):'''  m in A'''return "A"class B(A):def m(self):'''  m in B'''return "B" + Super(B, self).m()class C(A):def m(self):''' m in C '''return "C" + Super(C, self).m()class D(C):def m(self):''' m in D'''return "D" + Super(B, self).m()print(D().m()) # "DCBA"

现在，该代码就可以正确地运行，并且不会引发错误。