if inspect.isclass(func):
                F = spye.validation.StandardFunction(func.__init__, str(self.exp))
            elif hasattr(func, '__call__'):
                if hasattr(func.__call__, 'func_code'):
                    F = spye.validation.StandardFunction(func.__call__, str(self.exp))
                else:
                    F = BuiltinFunction(func, str(self.exp))
            else:
                self.print_exec_error("Trying to call uncallable object of type [%s]", type(func).__name__)
                return None