Question about isinstance()

[Mr.Rech]

Hi all,
I've read some thread about isinstance(), why it is considered harmful
and how you can achieve the same results using a coding style that
doesn't break polymorphism etc... Since I'm trying to improve my Python
knowledge, and I'm going to design a class hierarchy from scratch, I'd
like to have some feedback about this from those of you much more
experienced than me in OOP.

Suppose I'm writing a base class with an __eq__ special methods, using
isinstance() I would have wrote:

class foo(object):
   ...
   def __eq__(self, other):
      return isinstance(other, type(self)) and self.an_attribute ==
other.an__attribute

Now, avoiding isinstace() I've written the following code:

class foo(object):
   ...
   def __eq__(self, other):
      try:
         return self.an_attribute == other.an_attribute
      except AttributeError:
         return False

Any better way to write this method? Any suggestion?