python01

import reimport urlparseimport urllib2import timefrom datetime import datetimeimport robotparserimport Queue#链接爬虫'''一个链接爬虫需要考虑以下几个问题：1.网页可能不存在。就要用到try和except语句，捕获异常。2.临时性的错误。比如服务器过载返回的503 Service Unavailable错误。就要多尝试几次下载。3.代理。用户代理user_agent='wswp'。4.用正则表达式。'<a[^>]+href=["\'](.*?)["\']'.5.相对链接问题。如果是相对链接就应该创建绝对链接。urlparse.urljoin()6.重复爬取。爬取网页的时候，经常会出现将要爬取的网页中也有爬取过的链接，这样会造成不断循环。所以要建立一个URL管理器，管理爬取过的和未爬取的7.爬虫协议（robots.txt），所以要引入robotparser模块，以避免下载禁止爬取的URL8.用代理访问某个网站。9.下载限速。如果我们爬取网站的速度过快，就会面临被封禁或者服务器过载的风险。所以应当在两次下载之间添加延时。delay10.爬虫陷阱。最好设置一个爬取深度(max_depth)——记录到达当前网页经过了多少链接。'''def link_crawler(seed_url, link_regex=None, delay=5, max_depth=-1, max_urls=-1, headers=None, user_agent='wswp',                 proxy=None, num_retries=1):    """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 = Queue.deque([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    throttle = Throttle(delay)#下载限速    headers = headers or {}    if user_agent:        headers['User-agent'] = user_agent#用户代理    while crawl_queue:        url = crawl_queue.pop()#移除列表中的元素，并且返回该元素的值        # 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            throttle.wait(url)#延迟            html = download(url, headers, proxy=proxy, num_retries=num_retries)            links = []            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:', url#下载限速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):        domain = urlparse.urlparse(url).netloc#服务器位置        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()def download(url, headers, proxy, num_retries, data=None):    print 'Downloading:', url    request = urllib2.Request(url, data, headers)    opener = 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 urllib2.URLError as e:        print 'Download error:', e.reason        html = ''        if hasattr(e, 'code'):            code = e.code            if num_retries > 0 and 500 <= code < 600:                # retry 5XX HTTP errors                return download(url, headers, proxy, num_retries - 1, data)        else:            code = None    return htmldef 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_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 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)if __name__ == '__main__':    link_crawler('http://example.webscraping.com', '/(index|view)', delay=0, num_retries=1, user_agent='BadCrawler')    link_crawler('http://example.webscraping.com', '/(index|view)', delay=0, num_retries=1, max_depth=1,                 user_agent='GoodCrawler')