Types that define `__eq__` are unhashable?

Question

I had a strange bug when porting a feature to the Python 3.1 fork of my program. I narrowed it down to the following hypothesis:

In contrast to Python 2.x, in Python

Answer 1

This paragraph from http://docs.python.org/3.1/reference/datamodel.html#object.hash

If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = .__hash__. Otherwise the inheritance of __hash__() will be blocked, just as if __hash__ had been explicitly set to None.