code: def _clean(self, value):
          
          if value is None:
              return None
          elif self.type_ is None:
              return value
          elif self.check_type(value):
              return value
          elif self.is_type_castable:  # noqa
              return self.type_(value)
  
          error_fmt = "%s must be a %s, not a %s"
          error = error_fmt % (self.name, self.type_, type(value))
          raise TypeError(error)
docs: Validate and clean a candidate value for this field.