Parse matching expression in form key <op> value

            For example;
                vlan > 1
                node = FOO-BAR

def _parse_expr(self, part):
        """ Parse matching expression in form key <op> value

            For example;
                vlan > 1
                node = FOO-BAR
        """
        self._logger.debug("parsing expression: " + unicode(part))
        key, op, val = part

        success = True
        dictsql = {
                'operator': op,
                'val1': key,
                'val2': unicode(val),
                'interpretation': {
                    'string': key + op + val,
                    'interpretation': 'expression',
                    'attribute': key,
                    'operator': op,
                    'error': False
                }
            }

        if key in self.attributes:
            if type(self.attributes[key]) is list:
                if val not in self.attributes[key]:
                    dictsql['interpretation']['error'] = True
                    dictsql['interpretation']['error_message'] = 'invalid value'
                    success = False

        else:
            dictsql['interpretation']['error'] = True
            dictsql['interpretation']['error_message'] = 'unknown attribute'
            success = False

        return success, dictsql