def _default_return_columns(self):
        """
        Return a list of the model elements that does not include lookup functions
        or other functions that take parameters.
        """
        return_columns = []
        parsed_expr = []

        for key, value in self.components._namespace.items():
            if hasattr(self.components, value):
                sig = signature(getattr(self.components, value))
                # The `*args` reference handles the py2.7 decorator.
                if len(set(sig.parameters) - {'args'}) == 0:
                    expr = self.components._namespace[key]
                    if not expr in parsed_expr:
                        return_columns.append(key)
                        parsed_expr.append(expr)

        return return_columns
Return a list of the model elements that does not include lookup functions
        or other functions that take parameters.