In order to ensure the == and != operators behave consistently as expected (i.e. they should be negations of each other), care should be taken when implementing the __eq__ and __ne__ special methods.

In Python 3, if the __eq__ method is defined in a class while the __ne__ is not, then the != operator will automatically delegate to the __eq__ method in the expected way.

However, if the __ne__ method is defined without a corresponding __eq__ method, the == operator will still default to object identity (equivalent to the is operator), while the != operator will use the __ne__ method, which may be inconsistent.

Additionally, if the __ne__ method is defined on a superclass, and the subclass defines its own __eq__ method without overriding the superclass __ne__ method, the != operator will use this superclass __ne__ method, rather than automatically delegating to __eq__, which may be incorrect.

Ensure that when an __ne__ method is defined, the __eq__ method is also defined, and their results are consistent. In most cases, the __ne__ method does not need to be defined at all, as the default behavior is to delegate to __eq__ and negate the result.

In the following example, A defines a __ne__ method, but not an __eq__ method. This leads to inconsistent results between equality and inequality operators.

In the following example, C defines an __eq__ method, but its __ne__ implementation is inherited from B, which is not consistent with the equality operation.

Python Language Reference: object.__ne__, Comparisons.