code: def _retrieve_value(self, entity, default=None):
      
      return entity._values.get(self._name, default)
docs: Internal helper to retrieve the value for this Property from an entity.
  
      This returns None if no value is set, or the default argument if
      given.  For a repeated Property this returns a list if a value is
      set, otherwise None.  No additional transformations are applied.