python递归爬取整站链接，广度优先，主要用于监控网站是否发布了新的文章

除了js脚本跳转之外，无论是绝对链接，相对链接，还是有层级的相对链接（如../../xxx.htm），都可以很好的整理成完整的URL，其中leve是定义页面层级深度的。要想真正使用，还有好多路要走，比如：过滤页面类型，数据去重，多线程，批处理爬取多个网站等。

# coding:utf-8
# 网页url采集爬虫，给定网址，以及存储文件，将该网页内全部网址采集下，可指定文件存储方式
import requests, time
from lxml import etree
from urllib import parse

"""
    url:给定的url
    save_file_name:为url存储文件
"""

OLD_URL = 'http://www.research.pku.edu.cn'
level = 2; #递归层级，广度优先

def Redirect(url):
    try:
        res = requests.get(url, timeout=10)
        url = res.url
    except Exception as e:
        print("4", e)
        time.sleep(1)
    return url


def requests_for_url(url, save_file_name, file_model,features):
    global OLD_URL
    headers = {
        'pragma': "no-cache",
        'accept-encoding': "gzip, deflate, br",
        'accept-language': "zh-CN,zh;q=0.8",
        'upgrade-insecure-requests': "1",
        'user-agent': "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36",
        'accept': "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
        'cache-control': "no-cache",
        'connection': "keep-alive",
    }
    try:
        response = requests.request("GET", url, headers=headers)
        selector = etree.HTML(response.text, parser=etree.HTMLParser(encoding='utf-8'))
    except Exception as e:
        print("页面加载失败", e)

    return_set = set()
    with open(save_file_name, file_model) as f:
        try:
            context = selector.xpath('//a/@href')
            for i in context:
                try:
                    print("-------------" + i)
                    if i[:10] == "javascript":  # javascript跳转
                        continue
                    elif i[0] == "/":  # 相对路径跳转
                        # print i
                        # i = OLD_URL + i
                        i = parse.urljoin(url, i)  # 通过parse组装相对路径为绝对路径
                    elif i[0:4] == "http":  # 如果是完整的URL
                        i = i
                    elif i[0] == ".":  # 如果是有层级的相对路径
                        i = parse.urljoin(url, i)  # 通过parse组装相对路径为绝对路径
                    elif i[0] != ".":  # 如果是没有层级的相对路径
                        if url[-1] != "/":  # 如果不是“/”结尾
                            if url[8:].rfind("/") != -1:  # 如果连接中包含“/”
                                i = url[:url.rfind("/") + 1] + i  # 截取掉最后一个“/"之后字符串
                            else:
                                i = url + "/" + i
                        else:
                            i = url + i
                    if features in i: #不符合特征值，舍弃
                        f.write(i)
                        f.write("\n")
                        return_set.add(i)
                    # print(len(return_set))
                        print(len(return_set), i)
                except Exception as e:
                    print("1", e)
        except Exception as e:
            print("2", e)
    return return_set

def Recursion_GETurl(return_set,features):
    global level
    level -=1
    print("目前递归层级："+str(level))
    if level<0:
        return
    else:
        return_all = set()
        for value in return_set:
            # print(value)
            http = "http://"
            if http in value:
                return_set2 = requests_for_url(value, save_file_name, "a",features)
                return_all = return_all|return_set2 #合并SET()集
        Recursion_GETurl(return_all,features)

if __name__ == '__main__':
    # 网页url采集爬虫，给定网址，以及存储文件，将该网页内全部网址采集下，可指定文件存储方式
    url = "http://kxyjb.csu.edu.cn/"
    features = "csu.edu.cn" #特征值，URL不包含该字段的，舍弃。二级域名的情况下，应该定义为一级域名
    save_file_name = "url.txt"
    return_set = requests_for_url(url, save_file_name, "a",features)  # “a”:追加
    Recursion_GETurl(return_set,features)
    #对url.txt进行数据去重
    print(“终于爬完了，辛苦，伙计！”)