本文代码节选（略有改动）自《Python程序设计（第2版）》（董付国编著，清华大学出版社），没有使用scrapy爬虫框架，而是使用标准库urllib访问网页实现爬虫功能，如果网页包含感兴趣的关键词，就把这个网页保存成为本地文件，并且有效控制了爬取深度，避免爬遍互联网。

import sys

import re

import os

import urllib.request as lib

def craw_links(url, depth, keywords, processed):

'''url:the url to craw

depth:the current depth to craw

keywords:the tuple of keywords to focus

processed:the urls already crawled

'''

if url.startswith(('http://', 'https://')):

if url not in processed:

# mark this url as processed

processed.append(url)

else:

# avoid processing the same url again

return

print('Crawing '+url+'...')

with lib.urlopen(url) as fp:

# Python3 returns bytes

# so need to decode

contents = fp.read()

contents_decoded = contents.decode('UTF-8')

# form a regular expression

pattern = '|'.join(keywords)

# if this page contains certain keywords, save it to a file

flag = False

if pattern:

searched = re.search(pattern, contents_decoded)

else:

# if the keywords to filter is not given, save current page

flag = True

if flag or searched:

with open('craw\\'+url.replace(':','_').replace('/','_'), 'wb') as fp:

fp.write(contents)

# find all the links in the current page

links = re.findall('href="(.*?)"', contents_decoded)

# craw all links in the current page

for link in links:

# consider the relative path

if not link.startswith(('http://','https://')):

try:

index = url.rindex('/')

link = url[0:index+1]+link

except:

pass

# control the crawl depth

if depth>0 and link.endswith(('.htm','.html')):

craw_links(link, depth-1, keywords, processed)

if __name__ == '__main__':

processed = []

keywords = ('datetime','KeyWord2')

if not os.path.exists('craw') or not os.path.isdir('craw'):

os.mkdir('craw')

start_url = r'https://docs.python.org/3/library/index.html'

craw_links(start_url, 1, keywords, processed)