code: def read(self, entity=None, attrs=None, ignore=None, params=None):
          
          # read() should not change the state of the object it's called on, but
          # super() alters the attributes of any entity passed in. Creating a new
          # object and passing it to super() lets this one avoid changing state.
          if entity is None:
              if hasattr(self, 'smart_class_parameter'):
                  entity = type(self)(
                      self._server_config,
                      # pylint:disable=no-member
                      smart_class_parameter=self.smart_class_parameter,
                  )
              elif hasattr(self, 'smart_variable'):
                  entity = type(self)(
                      self._server_config,
                      # pylint:disable=no-member
                      smart_variable=self.smart_variable,
                  )
          if ignore is None:
              ignore = set()
          ignore.update(['smart_class_parameter', 'smart_variable'])
          return super(OverrideValue, self).read(entity, attrs, ignore, params)
docs: Provide a default value for ``entity``.
  
          By default, ``nailgun.entity_mixins.EntityReadMixin.read provides a
          default value for ``entity`` like so::
  
              entity = type(self)()
  
          However, :class:`OverrideValue` requires that an
          ``smart_class_parameter`` or ``smart_varaiable`` be provided, so this
          technique will not work. Do this instead::
  
              entity = type(self)(
                  smart_class_parameter=self.smart_class_parameter)
              entity = type(self)(smart_variable=self.smart_variable)