关于python源码中assert的疑问。

Question

def add_rules(self, rules):
        """Appends new rules to the router.
        :arg rules: a list of Rule instances (or tuples of arguments, which are
            passed to Rule constructor).
        """
        for rule in rules:
            if isinstance(rule, (tuple, list)):
                assert len(rule) in (2, 3, 4)
                if isinstance(rule[0], basestring_type):
                    rule = Rule(PathMatches(rule[0]), *rule[1:])
                else:
                    rule = Rule(*rule)

            self.rules.append(self.process_rule(rule))

这是一段tornado的源码，其中assert len(rule) in (2, 3, 4)的assert是什么意思？我查了python3.6文档（莫名python2.7文档打不开了），都没查到这个关键字..

Answer 1

断言后面的表达式是否为true