Understanding Python descriptors #4

snippet_4.py

class EmailField(object):
    __current_id = 0                    # unique id of the instance
    __email_dict = {}                   # dict containing all email values
    __dict_key = "__email_field_key"    # name of member attached to instances
        
    def __get__(self, instance, owner):
        if not hasattr(instance, self.__class__.__dict_key):
            return None
        
        # Retrieve the email value from the dict
        key_name = getattr(instance, self.__class__.__dict_key)
        return self.__class__.__email_dict.get(key_name, None)
 
    def __set__(self, instance, value):
        if not hasattr(instance, self.__class__.__dict_key):
            # We have a new instance with an email attached
            setattr(instance, self.__class__.__dict_key, self.__class__.__current_id)
            self.__class__.__current_id += 1
 
        validate_email(value)
        key_name = getattr(instance, self.__class__.__dict_key)
        
        # save the new email value in the dict
        self.__class__.__email_dict[key_name] = value