Python Data Structures and Algorithms

defaultdict

The defaultdict object is a subclass of dict and therefore they share methods and operations. It acts as a convenient way to initialize dictionaries. With a dict, Python will throw a KeyError when attempting to access a key that is not already in the dictionary. The defaultdict overrides one method, __missing__(key), and creates a new instance variable, default_factory. With defaultdict, rather than throw an error, it will run the function, supplied as the default_factory argument, which will generate a value. A simple use of defaultdict is to set default_factory to int and use it to quickly tally the counts of items in the dictionary, for example:

You will notice that if we tried to do this with an ordinary dictionary, we would ...