In order to conform to the object model, classes that define their own equality method should also define their own hash method, or be unhashable. If the hash method is not defined then the hash of the super class is used. This is unlikely to result in the expected behavior.

A class can be made unhashable by setting its __hash__ attribute to None.

In Python 3, if you define a class-level equality method and omit a __hash__ method then the class is automatically marked as unhashable.

When you define an __eq__ method for a class, remember to implement a __hash__ method or set __hash__ = None.

In the following example the Point class defines an equality method but no hash method. If hash is called on this class then the hash method defined for object is used. This is unlikely to give the required behavior. The PointUpdated class is better as it defines both an equality and a hash method. If Point was not to be used in dicts or sets, then it could be defined as UnhashablePoint below.

To comply fully with the object model this class should also define an inequality method (identified by a separate rule).

Python Language Reference: object.__hash__.

Python Glossary: hashable.