Return a function to parse numbers.

def _number_parser(str_to_number_func):
    """Return a function to parse numbers."""
    def _parse_number_value(element_text, state):
        value = None

        try:
            value = str_to_number_func(element_text)
        except (ValueError, TypeError):
            state.raise_error(InvalidPrimitiveValue,
                              'Invalid numeric value "{}"'.format(element_text))

        return value

    return _parse_number_value