How to use the asteval.astutils.ReturnedNone function in asteval

def on_return(self, node):  # ('value',)
        """Return statement: look for None, return special sentinal."""
        self.retval = self.run(node.value)
        if self.retval is None:
            self.retval = ReturnedNone
        return

self.raise_exc(None, msg=msg, lineno=self.lineno)

        save_symtable = self.__asteval__.symtable.copy()
        self.__asteval__.symtable.update(symlocals)
        self.__asteval__.retval = None
        retval = None

        # evaluate script of function
        for node in self.body:
            self.__asteval__.run(node, expr='<>', lineno=self.lineno)
            if len(self.__asteval__.error) > 0:
                break
            if self.__asteval__.retval is not None:
                retval = self.__asteval__.retval
                self.__asteval__.retval = None
                if retval is ReturnedNone:
                    retval = None
                break

        self.__asteval__.symtable = save_symtable
        symlocals = None
        return retval