code: def add_field(self, key, value, field):
          ''':meth:`add_field` must be used to add a field to an existing
          instance of Model. This method is required so that serialization of the
          data is possible. Data on existing fields (defined in the class) can be
          reassigned without using this method.
  
          '''
          self._extra[key] = field
          setattr(self, key, valuef add_field(self, key, value, field):
          ''':meth:`add_field` must be used to add a field to an existing
          instance of Model. This method is required so that serialization of the
          data is possible. Data on existing fields (defined in the class) can be
          reassigned without using this method.
  
          '''
          self._extra[key] = field
          setattr(self, key, value)
docs: :meth:`add_field` must be used to add a field to an existing
          instance of Model. This method is required so that serialization of the
          data is possible. Data on existing fields (defined in the class) can be
          reassigned without using this method.