How to use the asteval.get_ast_names function in asteval

def __set_expression(self, val):
        if val == '':
            val = None
        self._expr = val
        if val is not None:
            self.vary = False
        if not hasattr(self, '_expr_eval'):
            self._expr_eval = None
        if val is None:
            self._expr_ast = None
        if val is not None and self._expr_eval is not None:
            self._expr_eval.error = []
            self._expr_eval.error_msg = None
            self._expr_ast = self._expr_eval.parse(val)
            check_ast_errors(self._expr_eval)
            self._expr_deps = get_ast_names(self._expr_ast)

- 'omit' : drop missing data

        """
        # create ast evaluator, load custom functions
        self.asteval = Interpreter()
        for name in lineshapes.functions:
            self.asteval.symtable[name] = getattr(lineshapes, name, None)
        if init_script is not None:
            self.asteval.eval(init_script)

        # save expr as text, parse to ast, save for later use
        self.expr = expr.strip()
        self.astcode = self.asteval.parse(self.expr)

        # find all symbol names found in expression
        sym_names = get_ast_names(self.astcode)

        if independent_vars is None and 'x' in sym_names:
            independent_vars = ['x']
        if independent_vars is None:
            raise ValueError(self.idvar_missing % (self.expr))

        # determine which named symbols are parameter names,
        # try to find all independent variables
        idvar_found = [False]*len(independent_vars)
        param_names = []
        for name in sym_names:
            if name in independent_vars:
                idvar_found[independent_vars.index(name)] = True
            elif name not in param_names and name not in self.asteval.symtable:
                param_names.append(name)