code: def parse_arg(self, name, field, req, locations=None):
          
          location = field.metadata.get("location")
          if location:
              locations_to_check = self._validated_locations([location])
          else:
              locations_to_check = self._validated_locations(locations or self.locations)
  
          for location in locations_to_check:
              value = self._get_value(name, field, req=req, location=location)
              # Found the value; validate and return it
              if value is not missing:
                  return value
          return missing
docs: Parse a single argument from a request.
  
          .. note::
              This method does not perform validation on the argument.
  
          :param str name: The name of the value.
          :param marshmallow.fields.Field field: The marshmallow `Field` for the request
              parameter.
          :param req: The request object to parse.
          :param tuple locations: The locations ('json', 'querystring', etc.) where
              to search for the value.
          :return: The unvalidated argument value or `missing` if the value cannot
              be found on the request.