Scrapy：“str”对象没有属性“iter”

I added restrict_xpaths我的 scrapy 蜘蛛的规则，现在它立即失败：

2015-03-16 15:46:53+0000 [tsr] ERROR: Spider error processing <GET http://www.thestudentroom.co.uk/forumdisplay.php?f=143>
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/twisted/internet/base.py", line 800, in runUntilCurrent
        call.func(*call.args, **call.kw)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/twisted/internet/task.py", line 602, in _tick
        taskObj._oneWorkUnit()
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/twisted/internet/task.py", line 479, in _oneWorkUnit
        result = self._iterator.next()
      File "/Library/Python/2.7/site-packages/scrapy/utils/defer.py", line 57, in <genexpr>
        work = (callable(elem, *args, **named) for elem in iterable)
    --- <exception caught here> ---
      File "/Library/Python/2.7/site-packages/scrapy/utils/defer.py", line 96, in iter_errback
        yield next(it)
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spidermiddleware/offsite.py", line 26, in process_spider_output
        for x in result:
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spidermiddleware/referer.py", line 22, in <genexpr>
        return (_set_referer(r) for r in result or ())
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spidermiddleware/urllength.py", line 33, in <genexpr>
        return (r for r in result or () if _filter(r))
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spidermiddleware/depth.py", line 50, in <genexpr>
        return (r for r in result or () if _filter(r))
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spiders/crawl.py", line 73, in _parse_response
        for request_or_item in self._requests_to_follow(response):
      File "/Library/Python/2.7/site-packages/scrapy/contrib/spiders/crawl.py", line 52, in _requests_to_follow
        links = [l for l in rule.link_extractor.extract_links(response) if l not in seen]
      File "/Library/Python/2.7/site-packages/scrapy/contrib/linkextractors/lxmlhtml.py", line 107, in extract_links
        links = self._extract_links(doc, response.url, response.encoding, base_url)
      File "/Library/Python/2.7/site-packages/scrapy/linkextractor.py", line 94, in _extract_links
        return self.link_extractor._extract_links(*args, **kwargs)
      File "/Library/Python/2.7/site-packages/scrapy/contrib/linkextractors/lxmlhtml.py", line 50, in _extract_links
        for el, attr, attr_val in self._iter_links(selector._root):
      **File "/Library/Python/2.7/site-packages/scrapy/contrib/linkextractors/lxmlhtml.py", line 38, in _iter_links
        for el in document.iter(etree.Element):
    exceptions.AttributeError: 'str' object has no attribute 'iter'**

我不明白为什么会发生这个错误。

这是我的短片Spider:

import scrapy

from tutorial.items import DmozItem
from scrapy.contrib.spiders import CrawlSpider, Rule
from scrapy.contrib.linkextractors import LinkExtractor

class TsrSpider(CrawlSpider):
    name = 'tsr'
    allowed_domains = ['thestudentroom.co.uk']
    start_urls = ['http://www.thestudentroom.co.uk/forumdisplay.php?f=143']

    download_delay = 4
    user_agent = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:35.0) Gecko/20100101 Firefox/35.0'

    rules = (
        Rule(
            LinkExtractor(
                allow=('forumdisplay\.php\?f=143\&page=\d',),
                restrict_xpaths=("//li[@class='pager-page_numbers']/a/@href",))),

        Rule(
            LinkExtractor(
                allow=('showthread\.php\?t=\d+\&page=\d+',),
                restrict_xpaths=("//li[@class='pager-page_numbers']/a/@href",)), 
            callback='parse_link'),

        Rule(
            LinkExtractor(
                allow=('showthread\.php\?t=\d+',),
                restrict_xpaths=("//tr[@class='thread  unread    ']",)),
            callback='parse_link'),
        )

    def parse_link(self, response):
#           Iterate over posts.     
        for sel in response.xpath("//li[@class='post threadpost old   ']"):
            rating = sel.xpath(
            "div[@class='post-footer']//span[@class='score']/text()").extract()
            if not rating:
                rating = 0
            else:
                rating = rating[0]
            item = DmozItem()
            item['post'] = sel.xpath(
    "div[@class='post-content']/blockquote[@class='postcontent restore']/text()").extract()
            item['link'] = response.url
            item['topic'] = response.xpath(
    "//div[@class='forum-header section-header']/h1/span/text()").extract()
            item['rating'] = rating
            yield item

source: http://pastebin.com/YXdWvPgX http://pastebin.com/YXdWvPgX

有人可以帮我吗？错误在哪里？我找了好几天了！？

问题是restrict_xpaths should 指向元素- 直接链接或包含链接的容器，而不是属性：

rules = [
    Rule(LinkExtractor(allow='forumdisplay\.php\?f=143\&page=\d',
                       restrict_xpaths="//li[@class='pager-page_numbers']/a")),

    Rule(LinkExtractor(allow='showthread\.php\?t=\d+\&page=\d+',
                       restrict_xpaths="//li[@class='pager-page_numbers']/a"),
         callback='parse_link'),

    Rule(LinkExtractor(allow='showthread\.php\?t=\d+',
                       restrict_xpaths="//tr[@class='thread  unread    ']"),
         callback='parse_link'),
]

经过测试（为我工作）。

仅供参考，Scrapy 定义restrict_xpaths http://doc.scrapy.org/en/0.22/topics/link-extractors.html#scrapy.contrib.linkextractors.sgml.SgmlLinkExtractor作为“指向区域的表达式”：

restrict_xpaths(str 或 list) – 是一个 XPath（或 XPath 列表） 定义regions在响应中应提取链接的地方 从。如果给定，则仅扫描那些 XPath 选择的文本 用于链接。请参阅下面的示例。