Demo源码地址
https://github.com/CHUNL09/tornado/tree/master/demos/webspider
这个Demo的作用是用来获取特定URL的网页中的链接(链接是以特定URL作为开头的,比如设置了base_url=”http://www.baidu.com”,那么只会获取以”http://www.baidu.com开头的链接”)。代码如下:
#!/usr/bin/env python
import time
from datetime import timedeltatry: #python 2.7 适用
from HTMLParser import HTMLParser
from urlparse import urljoin, urldefrag
except ImportError:
from html.parser import HTMLParser
from urllib.parse import urljoin, urldefragfrom tornado import httpclient, gen, ioloop, queuesbase_url = 'http://www.tornadoweb.org/en/stable/'
concurrency = 10@gen.coroutine
def get_links_from_url(url):
"""Download the page at `url` and parse it for links. Returned links have had the fragment after `#` removed, and have been made
absolute so, e.g. the URL 'gen.html#tornado.gen.coroutine' becomes
'http://www.tornadoweb.org/en/stable/gen.html'.
"""
try:
response = yield httpclient.AsyncHTTPClient().fetch(url)
print('fetched %s' % url) html = response.body if isinstance(response.body, str) \
else response.body.decode()
urls = [urljoin(url, remove_fragment(new_url))
for new_url in get_links(html)]
except Exception as e:
print('Exception: %s %s' % (e, url))
raise gen.Return([]) raise gen.Return(urls)def remove_fragment(url):
pure_url, frag = urldefrag(url)
return pure_urldef get_links(html): # get all links in html page
class URLSeeker(HTMLParser):
def __init__(self):
HTMLParser.__init__(self)
self.urls = [] def handle_starttag(self, tag, attrs):
href = dict(attrs).get('href')
if href and tag == 'a':
self.urls.append(href) url_seeker = URLSeeker()
url_seeker.feed(html)
return url_seeker.urls@gen.coroutine
def main():
q = queues.Queue()
start = time.time()
fetching, fetched = set(), set() @gen.coroutine
def fetch_url():
current_url = yield q.get()
try:
if current_url in fetching:
return print('fetching %s' % current_url)
fetching.add(current_url)
urls = yield get_links_from_url(current_url)
fetched.add(current_url) for new_url in urls:
# Only follow links beneath the base URL
if new_url.startswith(base_url):
yield q.put(new_url) finally:
q.task_done() @gen.coroutine
def worker():
while True:
yield fetch_url() q.put(base_url) # Start workers, then wait for the work queue to be empty.
for _ in range(concurrency):
worker()
yield q.join(timeout=timedelta(seconds=300))
assert fetching == fetched
print('Done in %d seconds, fetched %s URLs.' % (
time.time() - start, len(fetched)))if __name__ == '__main__':
import logging
logging.basicConfig()
io_loop = ioloop.IOLoop.current()
io_loop.run_sync(main)
webspider
下面开始分析这个代码。
1 从程序的最终执行部分看起:
if __name__ == '__main__':
import logging
logging.basicConfig()
io_loop = ioloop.IOLoop.current()
io_loop.run_sync(main)
这里logging.basicConfig()貌似没有起作用,这个方法是在logging模块中用来设置日志的基本格式用的。这里显然没有用到。IOLoop.current()用来返回当前线程的IOloop. run_sync方法是用来启动IOLoop,运行,并且结束(Starts the IOLoop
, runs the given function, and stops the loop.)。
run_sync函数和tornado.gen.coroutine配合使用,主要是为了在mian函数中能够异步调用。Tornado官方给出了如下的使用示例:
@gen.coroutine
def main():
# do stuff...if __name__ == '__main__':
IOLoop.current().run_sync(main)
关于IOLoop.current()和IOLoop.instance()的区别请点击这里。
2 main函数。
首先,main函数前面带了@gen.coroutine装饰器,为了能够在main函数中实现异步调用。
@gen.coroutine
def main():
q = queues.Queue()
start = time.time()
fetching, fetched = set(), set() @gen.coroutine
def fetch_url():
current_url = yield q.get()
try:
if current_url in fetching:
return print('fetching %s' % current_url)
fetching.add(current_url)
urls = yield get_links_from_url(current_url) # 获取current_url页面中的link
fetched.add(current_url) for new_url in urls: # 对于子链接进行处理,只有符合条件的链接才会放入到queue中
# Only follow links beneath the base URL
if new_url.startswith(base_url):
yield q.put(new_url) finally:
q.task_done() #Indicate that a formerly enqueued task is complete. 表示get从queue中取出的任务已经完成 @gen.coroutine
def worker():
while True:
yield fetch_url() q.put(base_url) # Start workers, then wait for the work queue to be empty.
for _ in range(concurrency):
worker()
yield q.join(timeout=timedelta(seconds=300))
assert fetching == fetched
print('Done in %d seconds, fetched %s URLs.' % (
time.time() - start, len(fetched)))
line3 初始化了一个queue,这里使用的是tornado提供的queue(需要from tornado import queues ).
line5 初始化了两个集合fetching和fetched. fetching中存放正在处理的URL,而fetched中存放处理完成的URL。
line7-25 定义了函数fetch_url()主要是用来从queue中获取URL,并处理。
line27-30 定义了worker()函数,在其中使用了while True, 会不停的去yield fetch_url(). 这里while True是必须的,否则执行过一次的yield fetch_url()会hang住直到timeout.
line35-36 模拟并发效果,这里也可以取消for循环,但是实际结果消耗时间会大大多于并发的情况(可以自行测试实验)。
line37 q.join()的作用是block,直到queue中所有的任务都完成或者timeout.
line38 用断言来判断fetching 和fetched集合,正常情况下,两个集合中的URL数量应该是相等的。否则的话会raise一个断言的error出来。
3 其他定义的函数
代码如下:
@gen.coroutine
def get_links_from_url(url):
"""Download the page at `url` and parse it for links. Returned links have had the fragment after `#` removed, and have been made
absolute so, e.g. the URL 'gen.html#tornado.gen.coroutine' becomes
'http://www.tornadoweb.org/en/stable/gen.html'.
"""
try:
response = yield httpclient.AsyncHTTPClient().fetch(url)
print('fetched %s' % url) html = response.body if isinstance(response.body, str) \
else response.body.decode()
urls = [urljoin(url, remove_fragment(new_url))
for new_url in get_links(html)]
except Exception as e:
print('Exception: %s %s' % (e, url))
raise gen.Return([]) raise gen.Return(urls) def remove_fragment(url):
pure_url, frag = urldefrag(url)
return pure_url def get_links(html): # get all links in html page
class URLSeeker(HTMLParser):
def __init__(self):
HTMLParser.__init__(self)
self.urls = [] def handle_starttag(self, tag, attrs):
href = dict(attrs).get('href')
if href and tag == 'a':
self.urls.append(href) url_seeker = URLSeeker()
url_seeker.feed(html)
return url_seeker.urls
get_links_from_url函数
line 1-21定义的get_links_from_url函数,函数接收一个URL参数,并返回这个URL页面中所有的链接数量。使用URL获取页面内容这里使用的是tornado的httpclient中的方法httpclient.AsyncHTTPClient().fetch(). [也可以使用urllib.request.urlopen来抓取页面内容].
line15-16 分别调用了两个函数get_links和remove_fragment来获取新的URLs.
最终返回的是一个URL的列表。line 21 这里的raise gen.Return(urls) 可以直接替换为return urls,前者是旧版本tornado的用法。
get_links函数
line29-42定义了get_links函数,它接收html页面的内容,并将页面中的a标签的链接返回。实现方式是用HTMLParser。具体实现时要重写handle_starttag方法
remove_fragment 函数
line 24-26定义了remove_fragment函数,函数接收一个URL,并且会把URL中’#’后面的内容截掉,如:
>>> pure_url,frag = urldefrag("http://docs.python.org/2/reference/compound_stmts.html#the-with-statement #h1 #h2")
>>> pure_url
'http://docs.python.org/2/reference/compound_stmts.html'
>>> frag
'the-with-statement #h1 #h2'
小结
整体代码比较简洁,主要是使用了tornado的异步方式来获取。后续有时间会在这个基础上扩展下实现一个完整的爬虫。