Parse the primitive value within the XML element's attribute.

def _parse_attribute(
            self,
            element,  # type: ET.Element
            attribute,  # type: Text
            state  # type: _ProcessorState
    ):
        # type: (...) -> Any
        """Parse the primitive value within the XML element's attribute."""
        parsed_value = self._default
        attribute_value = element.get(attribute, None)

        if attribute_value is not None:
            parsed_value = self._parser_func(attribute_value, state)
        elif self.required:
            state.raise_error(
                MissingValue, 'Missing required attribute "{}" on element "{}"'.format(
                    self._attribute, element.tag
                )
            )

        return parsed_value