前言

上一篇中我們?cè)诰S基百科的內(nèi)部網(wǎng)站上隨機(jī)跳轉(zhuǎn)進(jìn)入文章類(lèi)網(wǎng)頁(yè)，而忽視外部網(wǎng)站鏈接贱鼻。本篇文章將處理網(wǎng)站的外部鏈接并試圖收集一些網(wǎng)站數(shù)據(jù)喻奥。和單個(gè)域名網(wǎng)站爬取不同，不同域名的網(wǎng)站結(jié)構(gòu)千差萬(wàn)別赖欣，這就意味我們的代碼需要更加的靈活以適應(yīng)不同的網(wǎng)站結(jié)構(gòu)屑彻。
因此，我們將代碼寫(xiě)成一組函數(shù)顶吮，這些函數(shù)組合起來(lái)就可以應(yīng)用在不同類(lèi)型的網(wǎng)絡(luò)爬蟲(chóng)需求社牲。

隨機(jī)跳轉(zhuǎn)外部鏈接

利用函數(shù)組，我們可以在50行左右滿足爬取外部網(wǎng)站的需求悴了。
示例代碼：

from urllib.request import urlopen
from bs4 import BeautifulSoup
import re
import datetime
import random
from urllib.parse import quote

pages = set()
random.seed(datetime.datetime.now())


''' 獲取一個(gè)網(wǎng)頁(yè)的所有互聯(lián)網(wǎng)鏈接'''

# 獲取網(wǎng)頁(yè)所有內(nèi)部鏈接
def get_internal_links(soup, include_url):
    internal_links = []
    # find all links that befin with a '/'
    print(include_url)
    for link in soup.find_all('a',
                              href=re.compile(r'^((/|.)*' + include_url + ')')):
        if link.attrs['href'] is not None:
            if link.attrs['href'] not in internal_links:
                internal_links.append(link.attrs['href'])
    return internal_links


# retrieves a list of all external links found on a page
#獲取網(wǎng)頁(yè)上所有外部鏈接
def get_external_links(soup, exclude_url):
    external_links = []
    # Finds all links that starts with 'http' or 'www' that do not contain the
    # current URL
    for link in soup.find_all('a',
                              href=re.compile(r'^(http|www)((?!' + exclude_url + ').)*$')):
        if link.attrs['href'] is not None:
            if link.attrs['href'] not in external_links:
                external_links.append(link.attrs['href'])
    return external_links


#拆分網(wǎng)址獲取主域名
def split_address(address):
    address_parts = address.replace('http://', '').split('/')
    return address_parts


#隨機(jī)外部鏈接跳轉(zhuǎn)
def get_random_external_link(starting_page):
    html = urlopen(starting_page)

    soup = BeautifulSoup(html, 'lxml')
    external_links = get_external_links(
        soup, split_address(starting_page)[0])  # find the domain URL
    if len(external_links) == 0:
        internal_links = get_internal_links(soup, starting_page)
        print(len(internal_links))
        return get_external_links(soup,
                                  internal_links[random.randint(0, len(internal_links) - 1)])
    else:
        return external_links[random.randint(0, len(external_links) - 1)]



hop_count = set()

#只跳轉(zhuǎn)外部鏈接搏恤，設(shè)置跳轉(zhuǎn)次數(shù)loop, 默認(rèn)跳轉(zhuǎn)5次
def follow_external_only(starting_site, loop=5):
    global hop_count
    external_link = get_random_external_link(
        quote(starting_site, safe='/:?='))
    print('Random external link is: ' + external_link)
    while len(hop_count) < loop:
        hop_count.add(external_link)
        print(len(hop_count))
        follow_external_only(external_link)



follow_external_only("http://www.baidu.com")

由于代碼沒(méi)有異常處理和反反爬蟲(chóng)處理，因此一定會(huì)報(bào)錯(cuò)湃交。由于跳轉(zhuǎn)是隨機(jī)的熟空，可以多運(yùn)行幾次，有興趣的可以根據(jù)每次的報(bào)錯(cuò)原因完善代碼搞莺。
輸出結(jié)果：

Random external link is: http://v.baidu.com/v?ct=301989888&rn=20&pn=0&db=0&s=25&ie=utf-8&word=
1
Random external link is: http://baishi.baidu.com/watch/6388818335201070269.html
2
Random external link is: http://v.baidu.com/tv/
3
Random external link is: http://player.baidu.com/yingyin.html
4
Random external link is: http://help.baidu.com/question?prod_en=player
5
Random external link is: http://home.baidu.com
[Finished in 6.3s]

抓取網(wǎng)頁(yè)上所有外部鏈接

把代碼寫(xiě)成函數(shù)的好處是可以簡(jiǎn)單地修改或者添加以滿足不同的需求而不會(huì)破壞代碼息罗。比如：
目的：爬取整個(gè)網(wǎng)頁(yè)所有外部鏈接并對(duì)每個(gè)鏈接標(biāo)記
我們可以添加如下函數(shù)：

# Collects a list of all external URLs found on the site
all_ext_links = set()
all_int_links = set()


def get_all_external_links(site_url):
    html = urlopen(site_url)
    soup = BeautifulSoup(html, 'lxml')
    print(split_address(site_url)[0])
    int
    internal_links = get_internal_links(soup, split_address(site_url)[0])
    external_links = get_external_links(soup, split_address(site_url)[0])
    for link in external_links:
        if link not in all_ext_links:
            all_ext_links.add(link)
            print(link)
    for link in internal_links:
        if link not in all_int_links:
            print('About to get link: ' + link)
            all_int_links.add(link)
            get_all_external_links(link)

# follow_external_only("http://www.baidu.com")
get_all_external_links('http://oreilly.com')

輸出結(jié)果如下：

oreilly.com
oreilly.com
https://cdn.oreillystatic.com/pdf/oreilly_high_performance_organizations_whitepaper.pdf
http://twitter.com/oreillymedia
http://fb.co/OReilly
https://www.linkedin.com/company/oreilly-media
https://www.youtube.com/user/OreillyMedia
About to get link: https://www.oreilly.com
https:
https:
https://www.oreilly.com
http://www.oreilly.com/ideas
https://www.safaribooksonline.com/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170601+nav
http://www.oreilly.com/conferences/
http://shop.oreilly.com/
http://members.oreilly.com
https://www.oreilly.com/topics
https://www.safaribooksonline.com/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170505+homepage+get+started+now
https://www.safaribooksonline.com/accounts/login/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170203+homepage+sign+in
https://www.safaribooksonline.com/live-training/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170201+homepage+take+a+live+online+course
https://www.safaribooksonline.com/learning-paths/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170201+homepage+follow+a+path
https://www.safaribooksonline.com/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170505+homepage+unlimited+access
http://www.oreilly.com/live-training/?view=grid
https://www.safaribooksonline.com/your-experience/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170201+homepage+safari+platform
https://www.oreilly.com/ideas/8-data-trends-on-our-radar-for-2017?utm_medium=referral&utm_source=oreilly.com&utm_campaign=lgen&utm_content=link+2017+trends
https://www.oreilly.com/ideas?utm_medium=referral&utm_source=oreilly.com&utm_campaign=lgen&utm_content=link+read+latest+articles
http://www.oreilly.com/about/
http://www.oreilly.com/work-with-us.html
http://www.oreilly.com/careers/
http://shop.oreilly.com/category/customer-service.do
http://www.oreilly.com/about/contact.html
http://www.oreilly.com/emails/newsletters/
http://www.oreilly.com/terms/
http://www.oreilly.com/privacy.html
http://www.oreilly.com/about/editorial_independence.html
About to get link: https://www.safaribooksonline.com/?utm_medium=content&utm_source=oreilly.com&utm_campaign=lgen&utm_content=20170601+nav
https:
https:
https://www.oreilly.com/
About to get link: https://www.oreilly.com/
https:
https:
About to get link: https://www.oreilly.com/topics
......

程序會(huì)一直循環(huán)下去直到達(dá)到python默認(rèn)的循環(huán)極限， 有興趣的朋友可以像上面的代碼一樣添加默認(rèn)循環(huán)限制loop=5才沧。