Parse the primitive value under the parent XML element.

def parse_from_parent(
            self,
            parent,  # type: ET.Element
            state  # type: _ProcessorState
    ):
        # type: (...) -> Any
        """Parse the primitive value under the parent XML element."""
        element = parent.find(self.element_path)

        if element is None and self.required:
            state.raise_error(
                MissingValue, 'Missing required element "{}"'.format(self.element_path)
            )
        elif element is not None:
            return self.parse_at_element(element, state)

        return _hooks_apply_after_parse(self._hooks, state, self._default)