Extending built-ins

One of the most interesting uses 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 on the list itself. We can do this using inheritance:

class ContactList(list):
    def search(self, name):
        '''Return all contacts that contain the search value
        in their name.'''
        matching_contacts = []
        for contact in self:
            if name in contact.name:
                matching_contacts.append(contact)
        return matching_contacts

class Contact:
 all_contacts = ContactList()
    
    def __init__(self, name, email):
        self.name = name
        self.email = email
        self.all_contacts.append(self)

Instead of instantiating a normal list as our class variable, we create a new ContactList class that extends the built-in list. Then we instantiate this subclass as our all_contacts list. We can test the new search functionality as follows:

>>> c1 = Contact("John A", "johna@example.net")
>>> c2 = Contact("John B", "johnb@example.net")
>>> c3 = Contact("Jenna C", "jennac@example.net")
>>> [c.name for c in Contact.all_contacts.search('John')]
['John A', 'John B']
>>> 

Are you wondering how we changed the built-in syntax [] into something we can inherit from? Creating an empty list with [] is actually a shorthand for creating an empty list using list(); the two syntaxes are identical:

>>> [] == list()
True

So, the list data type is like a class that we can extend, not unlike object.

As a second example, we can extend the dict class, which is the long way of creating a dictionary (the {:} syntax).

class LongNameDict(dict):
    def longest_key(self):
        longest = None
        for key in self:
            if not longest or len(key) > len(longest):
                longest = key
        return longest

This is easy to test in the interactive interpreter:

>>> longkeys = LongNameDict()
>>> longkeys['hello'] = 1
>>> longkeys['longest yet'] = 5
>>> longkeys['hello2'] = 'world'
>>> longkeys.longest_key()
'longest yet'

Most built-in types can be similarly extended. Commonly extended built-ins are object, list, set, dict, file, and str. Numerical types such as int and float are also occasionally inherited from.