def variables(self) -> list:
        """Variables."""
        try:
            assert self._variables is not None
        except (AssertionError, AttributeError):
            pattern = "|".join(map(re.escape, operators))
            keys = re.split(pattern, self.expression)
            indices = []
            for key in keys:
                if key in self.parent.variable_names:
                    indices.append(self.parent.variable_names.index(key))
            self._variables = [self.parent.variables[i] for i in indices]
        finally:
            return self._variables
Variables.