爬取策略

在爬虫系统中，待抓取URL队列是很重要的一部分。待抓取URL队列中的URL以什么样的顺序排列也是一个很重要的问题，因为这涉及到先抓取哪个页面，后抓取哪个页面。而决定这些URL排列顺序的方法，叫做抓取策略。下面重点介绍几种常见的抓取策略：

深度优先遍历策略

深度优先遍历策略是指网络爬虫会从起始页开始，一个链接一个链接跟踪下去，处理完这条线路之后再转入下一个起始页，继续跟踪链接。

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n6" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import re
import requests
​
header = {
"User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36"}
​
hrefre = "<a.href="(https?://.?)".*>"
​
def getPage(url):
'''
获取html
:param url:
:return: html源码
'''
html = requests.get(url, headers=header)
return html.text
​
def getUrl(url):
'''
获取url
:param url:
:return: URLList
'''
html = getPage(url)
urllist = re.findall(hrefre, html)
return urllist
​
def deepSpider(url, depth):
'''
深度爬虫
:param url:
:param depth:深度控制
:return:
'''
print("\t\t\t" * depthDict[url], "爬取了第%d级页面：%s" % (depthDict[url], url))
​
if depthDict[url] > depth:
return # 超出深度则跳出
sonlist = getUrl(url)
for i in sonlist:
if i not in depthDict:
depthDict[i] = depthDict[url] + 1 # 层级+1
deepSpider(i, depth)
​
if name == 'main':
depthDict = {} # 爬虫层级控制

起始url

startUrl = "https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&tn=baidu&wd=岛国邮箱"
depthDict[startUrl] = 1
deepSpider(startUrl, 4)
​</pre>

宽度优先遍历策略

宽度优先遍历策略的基本思路是，将新下载网页中发现的链接直接**待抓取URL队列的末尾。也就是指网络爬虫会先抓取起始网页中链接的所有网页，然后再选择其中的一个链接网页，继续抓取在此网页中链接的所有网页。还是以上面的图为例：遍历路径：A-B-C-D-E-F-G-H-I

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n9" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import re
import requests
​
header = {
"User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36"}
​
hrefre = "<a.href="(https?://.?)".>"
​
def getUrl(url):
'''
获取网页的全部url
:param url:
:return: url列表
'''
html = getPage(url)
'''
<a data-click="{}" href="http://www.baidu.com/" fasdf>...</a>
'''
urlre = "<a.href="(https?://.?)".>"
urllist = re.findall(urlre, html)
return urllist
​
def getPage(url):
'''
抓取网页html
:param url:
:return: HTML源码
'''
html = requests.get(url, headers=header).text
return html
​
def vastSpider(depth):
while len(urlList) > 0:
url = urlList.pop(0) # 弹出首个url
print("\t\t\t" * depthDict[url], "抓取了第%d级页面：%s" % (depthDict[url], url))
​
if depthDict[url] < depth:
sonList = getUrl(url)
for s in sonList:
if s not in depthDict: # 去重
depthDict[s] = depthDict[url] + 1
urlList.append(s)
​
if name == 'main':

去重

urlList = [] # url列表
​
depthDict = {}
starUrl = "https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&tn=baidu&wd=岛国邮箱"
depthDict[starUrl] = 1
urlList.append(starUrl)
vastSpider(4)
​</pre>

页面解析和数据提取

一般来讲对我们而言，需要抓取的是某个网站或者某个应用的内容，提取有用的价值。内容一般分为两部分，非结构化的数据 和 结构化的数据。

• 非结构化数据：先有数据，再有结构，

• 结构化数据：先有结构、再有数据 ####不同类型的数据，我们需要采用不同的方式来处理。

• 非结构化的数据处理

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="文本、电话号码、邮箱地址" contenteditable="true" cid="n19" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">HTML</pre>

• 结构化的数据处理

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="JSON 文件" contenteditable="true" cid="n23" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">JSON
XML</pre>

Beautiful Soup 4.2.0 文档

https://www.crummy.com/software/BeautifulSoup/bs4/doc/index.zh.html

示例：爬取前程无忧招聘岗位数量

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n29" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">from bs4 import BeautifulSoup
import requests
​
def download(url):
headers = {"User-Agent": "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0);"}
response = requests.get(url, headers=headers)
html = response.content.decode('gbk')

soup = BeautifulSoup(html, 'lxml')

获取岗位数量的多种查找方式

方式1： 使用find_all

jobnum = soup.find_all('div', class_='rt')
print(jobnum[0].text)

方式2： 使用select

jobnum = soup.select('.rt')[0].string
print(jobnum.strip()) # 去掉首尾空格
​

方式3：正则匹配re

jobnum_re = '<div class="rt">(.*?)</div>'

jobnum_comp = re.compile(jobnum_re, re.S)

jobnums = jobnum_comp.findall(html)

print(jobnums[0])

​
download(url = "https://search.51job.com/list/000000,000000,0000,00,9,99,python,2,1.html?lang=c&stype=&postchannel=0000&workyear=99&cotype=99&degreefrom=99&jobterm=99&companysize=99&providesalary=99&lonlat=0%2C0&radius=-1&ord_field=0&confirmdate=9&fromType=&dibiaoid=0&address=&line=&specialarea=00&from=&welfare=")
​</pre>

示例：爬取股票基金

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n31" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import urllib
from urllib import request
from bs4 import BeautifulSoup
​
stockList = []
​
def download(url):
headers = {"User-Agent": "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0);"}
request = urllib.request.Request(url, headers=headers) # 请求，修改，模拟http.
data = urllib.request.urlopen(request).read() # 打开请求，抓取数据

soup = BeautifulSoup(data, "html5lib", from_encoding="gb2312")
mytable = soup.select("#datalist")
for line in mytable[0].find_all("tr"):
print(line.get_text()) # 提取每一个行业
print(line.select("td:nth-of-type(3)")[0].text) # 提取具体的某一个
​
if name == 'main':
download("http://quote.stockstar.com/fund/stock_3_1_2.html")
​</pre>

练习：爬取腾讯岗位说明

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n33" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import urllib
from urllib import request
from bs4 import BeautifulSoup
​
def download(url):
headers = {"User-Agent": "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0);"}
request = urllib.request.Request(url, headers=headers) # 请求，修改，模拟http.
data = urllib.request.urlopen(request).read() # 打开请求，抓取数据

soup = BeautifulSoup(data, "html5lib")
print(soup)
data = soup.find_all("ul", class_="squareli")
for dataline in data:
for linedata in dataline.find_all("li"):
print(linedata.string)

data = soup.select('ul[class="squareli"]')
for dataline in data:
for linedata in dataline.select("li"):
print(linedata.get_text())
​
download("https://hr.tencent.com/position_detail.php?id=43940&keywords=%E7%88%AC%E8%99%AB&tid=0&lid=0")</pre>

练习：获取腾讯岗位列表

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n35" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import urllib
from urllib import request
from bs4 import BeautifulSoup
​
def download(url):
headers = {"User-Agent": "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0);"}
request = urllib.request.Request(url, headers=headers) # 请求，修改，模拟http.
data = urllib.request.urlopen(request).read() # 打开请求，抓取数据
soup = BeautifulSoup(data, "lxml")

data = soup.find_all("table", class_="tablelist")
for line in data[0].find_all("tr", class_=["even", "odd"]):
print(line.find_all("td")[0].a["href"])
for data in line.find_all("td"):
print(data.string)
​
download("https://hr.tencent.com/position.php?keywords=python&lid=0&tid=0#a")
​</pre>

存入数据库

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="true" cid="n37" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">import pymysql
​

存入数据库

def save_job(tencent_job_list):
​

连接数据库

db = pymysql.connect(host="127.0.0.1", port=3306, user='root', password="root",database='tencent1', charset='utf8')

游标

cursor = db.cursor()

遍历，插入job

for job in tencent_job_list:
sql = 'insert into job(name, address, type, num) VALUES("%s","%s","%s","%s") ' % (job["name"], job["address"], job["type"], job["num"])
cursor.execute(sql)
db.commit()
​
cursor.close()
db.close()</pre>