Subclassing built-in types

Subclassing built-in types in Python is pretty straightforward. A built-in type called object is a common ancestor for all built-in types as well as all user-defined classes that have no explicit parent class specified. Thanks to this, every time a class that behaves almost like one of the built-in types needs to be implemented, the best practice is to subtype it.

Now, we will show you the code for a class called distinctdict, which uses this technique. It is a subclass of the usual Python dict type. This new class behaves in most ways like an ordinary Python dict. But instead of allowing multiple keys with the same value, when someone tries to add a new entry with an identical value, it raises a ValueError subclass with a help message:

class DistinctError(ValueError):
    """Raised when duplicate value is added to a distinctdict."""

class distinctdict(dict):
    """Dictionary that does not accept duplicate values."""
    def __setitem__(self, key, value):
   ...