Return a unicode object with the Gremlin representation of this expression.

def to_gremlin(self):
        """Return a unicode object with the Gremlin representation of this expression."""
        self.validate()

        local_object_name = self.get_local_object_gremlin_name()

        if self.field_name == '@this':
            return local_object_name

        if '@' in self.field_name:
            return u'{}[\'{}\']'.format(local_object_name, self.field_name)
        else:
            return u'{}.{}'.format(local_object_name, self.field_name)