In 2.6a, seems like the __hash__ implementation and __eq__ must be
defined together, in the same class.  See also #1549.  Ensuring that a
__hash__ implementation isn't being pulled from a builtin type is
probably a sufficient check...?

>>> class Base(object):
...     def __init__(self, name):
...         self.name = name
...     def __eq__(self, other):
...         return self.name == other.name
...     def __hash__(self):
...         return hash(self.name)
... 
>>> class Extended(Base):
...     def __eq__(self, other):
...         print "trace: __eq__ called"
...         return Base.__eq__(self, other)
... 
>>> hash(Base('b1'))
603887253
>>> hash(Extended('e1'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'Extended'