ex.py

class DBInterfaceMeta(type):
    # we use __init__ rather than __new__ here because we want
    # to modify attributes of the class *after* they have been
    # created
    def __init__(cls, name, bases, dct):
        if not hasattr(cls, 'registry'):
            # this is the base class.  Create an empty registry
            print "Inside if"+str(cls)
            print name
            print bases
            print dct
            cls.registry = {}
        else:
            # this is a derived class.  Add cls to the registry
            print "inside else, cls:"+str(cls)
            print name
            print bases
            print dct
            
            interface_id = name.lower()
            cls.registry[interface_id] = cls
            print "cls.registry :"
            print cls.registry
        super(DBInterfaceMeta, cls).__init__(name, bases, dct)
    
class DBInterface(object):
    __metaclass__ = DBInterfaceMeta
    model = None
    @classmethod
    def add_field(cls, name):
        cls.model = name
        
    
print(DBInterface.registry)

class FirstInterface(DBInterface):
    FirstInterface.add_field("User")

class SecondInterface(DBInterface):
    pass

class SecondInterfaceModified(SecondInterface):
    pass

print(DBInterface.registry)