Why overidding is required for a dict class in Python

Question

I was reading source code of a banking application and the class bank is defined below:

class Bank(object):
    """ the bank class contains all the bank operations """

    def __init__(self, name):
        """ instantiate the class """
        self.name = str(name)
        self.customers = Customers()

Now self.customers is another instance of Customers class which is defined below:

class Customers(dict):
    """ the customers class extends the dictionary object """

    def __setitem__(self, key, item):
        self.__dict__[key] = item

    def __getitem__(self, key):
        return self.__dict__[key]

    def __repr__(self):
        return repr(self.__dict__)

    def __len__(self):
        return len(self.__dict__)

    def __delitem__(self, key):
        del self.__dict__[key]

    def keys(self):
        return self.__dict__.keys()

    def values(self):
        return self.__dict__.values()

    def __cmp__(self, dict):
        return cmp(self.__dict__, dict)

    def __contains__(self, item):
        return item in self.__dict__

    def add(self, key, value):
        self.__dict__[key] = value

    def __iter__(self):
        return iter(self.__dict__)

    def __call__(self):
        return self.__dict__

    def __unicode__(self):
        return unicode(repr(self.__dict__))

• As per my understanding we override a function when a new functionality is added or its behaviour is changed from previous function. Why we are overriding the functions of dict in Customer class. Can't we simply just use self.customers = dict() ? Since we are not adding anything new here.

Answer 1

The class is more than just a dict; it supports attribute access for the keys too, because it delegates all dictionary access to the instance __dict__ attribute, which contains all attributes.

Demo:

>>> c = Customers()
>>> c.foo = 'bar'
>>> c
{'foo': 'bar'}
>>> c['foo']
'bar'

You can't do that with a regular dictionary. The implementation you found is a rather elaborate version of the answers to this question: Accessing dict keys like an attribute in Python?