by default; with them, all objects compare unequal (except with themselves)

and ``x.__hash__()`` returns ``id(x)``.

Classes which inherit a :meth:`__hash__` method from a parent class but

change the meaning of :meth:`__cmp__` or :meth:`__eq__` such that the hash

value returned is no longer appropriate (e.g. by switching to a value-based

concept of equality instead of the default identity based equality) can

explicitly flag themselves as being unhashable by setting

``__hash__ = None`` in the class definition. Doing so means that not only

will instances of the class raise an appropriate :exc:`TypeError` when

a program attempts to retrieve their hash value, but they will also be

correctly identified as unhashable when checking

``isinstance(obj, collections.Hashable)`` (unlike classes which define

their own :meth:`__hash__` to explicitly raise :exc:`TypeError`).

If a class that overrrides :meth:`__cmp__` or :meth:`__eq__` needs to

retain the implementation of :meth:`__hash__` from a parent class,

the interpreter must be told this explicitly by setting

``__hash__ = <ParentClass>.__hash__``. Otherwise the inheritance of

:meth:`__hash__` will be blocked, just as if :attr:`__hash__` had been

explicitly set to :const:`None`.