Evaluate text and return (obj, valid)
        where *obj* is the object represented by *text*
        and *valid* is True if object evaluation did not raise any exception

def eval(self, text):
        """
        Evaluate text and return (obj, valid)
        where *obj* is the object represented by *text*
        and *valid* is True if object evaluation did not raise any exception
        """
        assert is_text_string(text)
        try:
            return eval(text, self.locals), True
        except:
            return None, False