A hashable class has an __eq__ method, and a __hash__ method that agrees with equality. When a hash method is defined, an equality method should also be defined; otherwise object identity is used for equality comparisons which may not be intended.

Note that defining an __eq__ method without defining a __hash__ method automatically makes the class unhashable in Python 3. (even if a superclass defines a hash method).

If a __hash__ method is defined, ensure a compatible __eq__ method is also defined.

To explicitly declare a class as unhashable, set __hash__ = None, rather than defining a __hash__ method that always raises an exception. Otherwise, the class would be incorrectly identified as hashable by an isinstance(obj, collections.abc.Hashable) call.

In the following example, the A class defines an hash method but no equality method. Equality will be determined by object identity, which may not be the expected behaviour.

Python Language Reference: object.__hash__.

Python Glossary: hashable.