为什么python3递归函数会返回null答案

【问题标题】：why would python3 recursive function return null为什么python3递归函数会返回null
【发布时间】：2018-11-01 01:17:48
【问题描述】：

我有这个功能，当达到速率限制时会再次调用自己。它最终应该会成功并返回工作数据。它正常工作，然后速率限制按预期工作，最后当数据恢复正常时，我得到：

TypeError: 'NoneType' 对象不可下标

def grabPks(pageNum):
    # cloudflare blocks bots...use scraper library to get around this or build your own logic to store and use a manually generated cloudflare session cookie... I don't care ????

    req = scraper.get("sumurl.com/"+str(pageNum)).content
    if(req == b'Rate Limit Exceeded'):
        print("adjust the rate limiting because they're blocking us :(")
        manPenalty = napLength * 3
        print("manually sleeping for {} seconds".format(manPenalty))
        time.sleep(manPenalty)
        print("okay let's try again... NOW SERVING {}".format(pageNum))
        grabPks(pageNum)
    else:
        tree = html.fromstring(req)
        pk = tree.xpath("/path/small/text()")
        resCmpress = tree.xpath("path/a//text()")
        resXtend = tree.xpath("[path/td[2]/small/a//text()")
        balance = tree.xpath("path/font//text()")
        return pk, resCmpress, resXtend, balance

我试图将 return 移到 else 范围之外，但它会抛出：

UnboundLocalError: 赋值前引用了局部变量 'pk'

【问题讨论】：

标签： python-3.x recursion

【解决方案1】：

如果您的顶级grabPks 受到速率限制，则不会返回任何内容。

想一想：

致电grabPks()
您的速率受限，因此您进入 if 语句并再次调用 grabPks()。
这次它成功了，所以grabPks() 将值返回给它上面的函数。
第一个函数现在退出 if 语句，到达函数末尾并且不返回任何内容。

尝试在 if 块内使用 return grabPks(pageNum)。

【讨论】：

【解决方案2】：

好吧...我需要返回grabPKs 让它玩得更好...：

def grabPks(pageNum):
    # cloudflare blocks bots...use scraper library to get around this or build your own logic to store and use a manually generated cloudflare session cookie... I don't care ?

    req = scraper.get("sumurl.com/"+str(pageNum)).content
    if(req == b'Rate Limit Exceeded'):
        print("adjust the rate limiting because they're blocking us :(")
        manPenalty = napLength * 3
        print("manually sleeping for {} seconds".format(manPenalty))
        time.sleep(manPenalty)
        print("okay let's try again... NOW SERVING {}".format(pageNum))
        return grabPks(pageNum)
    else:
        tree = html.fromstring(req)
        pk = tree.xpath("/path/small/text()")
        resCmpress = tree.xpath("path/a//text()")
        resXtend = tree.xpath("[path/td[2]/small/a//text()")
        balance = tree.xpath("path/font//text()")
        return pk, resCmpress, resXtend, balance

【讨论】：