Extending Built-in Functions

One interesting use of this kind of inheritance is adding functionality to built-in classes. In the Contact class seen earlier, we are adding contacts to a list of all contacts. What if we also wanted to search that list by name? Well, we could add a method on the Contact class to search it, but it feels like this method actually belongs to the list itself.

Example 1

The following example shows how we can do this using inheritance from a built-in type. In this case, we’re using the list type. We’re going to inform mypy that our list is only of instances of the Contact class by using list["Contact"]. For this syntax to work in Python, we need to also import the annotations module from the __future__ package. The definitions look like this: