磁盘缓存

#-*- coding:UTF-8 -*-#原来创建对象时或者是调用类以外的方法时提示没有定义是因为这些类或方法的位置不应该放在主函数后面，而应该放在主函数前面import urlparseimport urllib2import randomimport timefrom datetime import datetime, timedeltaimport socketimport robotparserimport csvimport reimport lxml.htmlDEFAULT_AGENT = 'wswp'DEFAULT_DELAY = 5DEFAULT_RETRIES = 1DEFAULT_TIMEOUT = 60def link_crawler(seed_url, link_regex=None, max_depth=-1, max_urls=-1, user_agent='wswp',  scrape_callback=None, cache=None):    """Crawl from the given seed URL following links matched by link_regex    """    # the queue of URL's that still need to be crawled 双向队列里面存储url    crawl_queue = [seed_url]    # the URL's that have been seen and at what depth    seen = {seed_url: 0}    # track how many URL's have been downloaded    num_urls = 0    rp = get_robots(seed_url)#获取robots.txt    D = Downloader(DEFAULT_DELAY, DEFAULT_AGENT, None, DEFAULT_RETRIES, DEFAULT_TIMEOUT, None, None)    while crawl_queue:        url = crawl_queue.pop()#移除列表中的元素，并且返回该元素的值        depth = seen[url]        # check url passes robots.txt restrictions        if rp.can_fetch(user_agent, url):#确定指定的用户代理是否允许访问网页            html = D(url)            links = []            if scrape_callback:                links.extend(scrape_callback(url, html) or [])            depth = seen[url]            if depth != max_depth:                # can still crawl further                if link_regex:                    # filter for links matching our regular expression                    links.extend(link for link in get_links(html) if re.match(link_regex, link))                for link in links:                    link = normalize(seed_url, link) #返回绝对链接                    # check whether already crawled this link                    if link not in seen:                        seen[link] = depth + 1                        # check link is within same domain                        if same_domain(seed_url, link):                            # success! add this new link to queue                            crawl_queue.append(link)            # check whether have reached downloaded maximum            num_urls += 1            if num_urls == max_urls:                break        else:            print 'Blocked by robots.txt:', urldef get_robots(url):    """Initialize robots parser for this domain    """    rp = robotparser.RobotFileParser()    rp.set_url(urlparse.urljoin(url, '/robots.txt'))#绝对链接    rp.read()    return rpdef normalize(seed_url, link):    """Normalize this URL by removing hash and adding domain    """    link, _ = urlparse.urldefrag(link)  # remove hash to avoid duplicates urldefrag(url)将url分解成去掉fragment的新url和去掉的fragment的二元组    return urlparse.urljoin(seed_url, link)#绝对链接def same_domain(url1, url2):    """Return True if both URL's belong to same domain    """    #将url分解成部件的6元组    return urlparse.urlparse(url1).netloc == urlparse.urlparse(url2).netlocdef get_links(html):    """Return a list of links from html    """    # a regular expression to extract all links from the webpage    #re.compile()函数将正则表达式的字符串形式编译为Pattern实例，然后使用Pattern实例处理文本并获得匹配结果（一个Match实例），最后使用Match实例获得信息，进行其他的操作。    webpage_regex = re.compile('<a[^>]+href=["\'](.*?)["\']', re.IGNORECASE)    # list of all links from the webpage    return webpage_regex.findall(html)class Downloader:    def __init__(self, delay=DEFAULT_DELAY, user_agent=DEFAULT_AGENT, proxies=None, num_retries=DEFAULT_RETRIES,                 timeout=DEFAULT_TIMEOUT, opener=None, cache=None):        socket.setdefaulttimeout(timeout)        self.throttle = Throttle(delay)        self.user_agent = user_agent        self.proxies = proxies        self.num_retries = num_retries        self.opener = opener        self.cache = cache    def __call__(self, url):        result = None        if self.cache:            try:                result = self.cache[url]            except KeyError:                # url is not available in cache                pass            else:                if self.num_retries > 0 and 500 <= result['code'] < 600:                    # server error so ignore result from cache and re-download                    result = None        if result is None:            # result was not loaded from cache so still need to download            self.throttle.wait(url)            proxy = random.choice(self.proxies) if self.proxies else None            headers = {'User-agent': self.user_agent}            result = self.download(url, headers, proxy=proxy, num_retries=self.num_retries)            if self.cache:                # save result to cache                self.cache[url] = result        return result['html']    def download(self, url, headers, proxy, num_retries, data=None):        print 'Downloading:', url        request = urllib2.Request(url, data, headers or {})        opener = self.opener or urllib2.build_opener()        if proxy:            proxy_params = {urlparse.urlparse(url).scheme: proxy}            opener.add_handler(urllib2.ProxyHandler(proxy_params))        try:            response = opener.open(request)            html = response.read()            code = response.code        except Exception as e:            print 'Download error:', str(e)            html = ''            if hasattr(e, 'code'):                code = e.code                if num_retries > 0 and 500 <= code < 600:                    # retry 5XX HTTP errors                    return self._get(url, headers, proxy, num_retries - 1, data)            else:                code = None        return {'html': html, 'code': code}class ScrapeCallback:    def __init__(self):        self.writer = csv.writer(open('countries.csv', 'w'))        self.fields = ('area', 'population', 'iso', 'country', 'capital',          'continent', 'tld', 'currency_code', 'currency_name',          'phone', 'postal_code_format', 'postal_code_regex', 'languages')        self.writer.writerow(self.fields)    def __call__(self, url, html):        if re.search('view', url):            tree = lxml.html.fromstring(html)            row = []            for field in self.fields:                row.append(tree.cssselect('table > tr#places_{}__row > td.w2p_fw'.format(field))[0].text_content())                self.writer.writerow(row)class Throttle:    """Throttle downloading by sleeping between requests to same domain    """    def __init__(self, delay):        # amount of delay between downloads for each domain        self.delay = delay        # timestamp of when a domain was last accessed        self.domains = {}    def wait(self, url):        """Delay if have accessed this domain recently        """        domain = urlparse.urlsplit(url).netloc        last_accessed = self.domains.get(domain)        if self.delay > 0 and last_accessed is not None:            sleep_secs = self.delay - (datetime.now() - last_accessed).seconds            if sleep_secs > 0:                time.sleep(sleep_secs)        self.domains[domain] = datetime.now()if __name__ == '__main__':    link_crawler('http://example.webscraping.com', '/(index|view)', scrape_callback=ScrapeCallback())