最近在看python网络数据采集这本书，并跟着书中的章节进行练习，在练习第五章，六度空间游戏中，其中递归函数看起来有点蒙 后经过在笔记本上将递归过程罗列下来，才理清头绪，源代码如下
from bs4 import BeautifulSoup
from urllib.request import urlopen
import re
import pymysql
conn=pymysql.connect(host='127.0.0.1', user='root', passwd='zhaimingyu', db='mysql', charset='utf8')
cur = conn.cursor()
cur.execute("USE wikipedia")
def insertPageIfNotExists(url):
cur.execute("SELECT *FROM pages WHERE url = %s", (url))
if cur.rowcount == 0:
cur.execute("INSERT INTO pages (url) VALUES (%s)", (url))
conn.commit()
return cur.lastrowid
else:
return cur.fetchone()[0]
def insertLink(fromPageId, toPageId):
cur.execute("SELECT * FROM link WHERE fromPageId = %s AND toPageId = %s", ((fromPageId,(toPageId))))
if cur.rowcount == 0:
cur.execute("INSERT INTO link (fromPageId, toPageId) VALUES (%s, %s)", ((fromPageId, (toPageId))))
conn.commit()
pages = set()
def getLinks(pageUrl, recursionLevel):
global pages
if recursionLevel > 4:
return ;
pageId = insertPageIfNotExists(pageUrl)
html = urlopen("https://en.wikipedia.org"+pageUrl)
bsObj = BeautifulSoup(html, "html.parser")
for link in bsObj.findAll("a", href=re.compile("^(/wiki/)((?!:).)*$")):
insertLink(pageId, insertPageIfNotExists(link.attrs['href']))
if link.attrs['href'] not in pages:
newPage = link.attrs['href']
pages.add(newPage)
getLinks(newPage, recursionLevel+1)
getLinks("/wiki/Kevin_Bacon", 0)
cur.close()
conn.close()
其中比较绕的部分是getLinks中，使用了for循环，在for循环中又调用了getLinks函数，这样实现的递归函数
def getLinks(pageUrl, recursionLevel):
global pages
if recursionLevel > 4:
return ;
pageId = insertPageIfNotExists(pageUrl)
html = urlopen("https://en.wikipedia.org"+pageUrl)
bsObj = BeautifulSoup(html, "html.parser")
for link in bsObj.findAll("a", href=re.compile("^(/wiki/)((?!:).)*$")):
insertLink(pageId, insertPageIfNotExists(link.attrs['href']))
if link.attrs['href'] not in pages:
newPage = link.attrs['href']
pages.add(newPage)
getLinks(newPage, recursionLevel+1)
getLinks("/wiki/Kevin_Bacon", 0)
通过下列代码得出的结果可以清晰的反映出程序的执行过程
list0=[1,2,3]
list1=[11,22,33]
list2=[111,222,333]
def test(rec):
k=rec
if rec>2:
return;
if rec==0:
rec=list0
if rec==1:
rec=list1
if rec==2:
rec=list2
for i in rec:
print("我是第 %d 层循环中的 %d"%(k,i))
test(k+1)
test(0)
上面函数返回结果如下
拓扑图如下
说明：程序执行过程按照上图1-9过程，之后以此类推
总结，执行函数执行for循环中的首个变量，当递归数超过限定数值，取并列for循环中所有值，返回到上一个父元素取for中所有值，以此类推
