Tornado Demo1---webspider分析

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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 timedelta

try: #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, urldefrag

from tornado import httpclient, gen, ioloop, queues

base_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_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


@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:

1 if __name__ == \'__main__\':
2     import logging
3     logging.basicConfig()
4     io_loop = ioloop.IOLoop.current() 
5     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。

 1 @gen.coroutine
 2 def main():
 3     q = queues.Queue()
 4     start = time.time()
 5     fetching, fetched = set(), set()
 6 
 7     @gen.coroutine
 8     def fetch_url():
 9         current_url = yield q.get()
10         try:
11             if current_url in fetching:
12                 return
13 
14             print(\'fetching %s\' % current_url)
15             fetching.add(current_url)
16             urls = yield get_links_from_url(current_url)  # 获取current_url页面中的link
17             fetched.add(current_url)
18 
19             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
20                 # Only follow links beneath the base URL
21                 if new_url.startswith(base_url):
22                     yield q.put(new_url)
23 
24         finally:
25             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
26 
27     @gen.coroutine
28     def worker():
29         while True:
30             yield fetch_url()
31 
32     q.put(base_url)
33 
34     # Start workers, then wait for the work queue to be empty.
35     for _ in range(concurrency):
36         worker()
37     yield q.join(timeout=timedelta(seconds=300)) 
38     assert fetching == fetched
39     print(\'Done in %d seconds, fetched %s URLs.\' % (
40         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:

 1 @gen.coroutine
 2 def get_links_from_url(url):
 3     """Download the page at `url` and parse it for links.
 4 
 5     Returned links have had the fragment after `#` removed, and have been made
 6     absolute so, e.g. the URL \'gen.html#tornado.gen.coroutine\' becomes
 7     \'http://www.tornadoweb.org/en/stable/gen.html\'.
 8     """
 9     try:
10         response = yield httpclient.AsyncHTTPClient().fetch(url)
11         print(\'fetched %s\' % url)
12 
13         html = response.body if isinstance(response.body, str) \\
14             else response.body.decode()
15         urls = [urljoin(url, remove_fragment(new_url))
16                 for new_url in get_links(html)]
17     except Exception as e:
18         print(\'Exception: %s %s\' % (e, url))
19         raise gen.Return([])
20 
21     raise gen.Return(urls)
22 
23 
24 def remove_fragment(url):
25     pure_url, frag = urldefrag(url)
26     return pure_url
27 
28 
29 def get_links(html):  # get all links in html page
30     class URLSeeker(HTMLParser):
31         def __init__(self):
32             HTMLParser.__init__(self)
33             self.urls = []
34 
35         def handle_starttag(self, tag, attrs):
36             href = dict(attrs).get(\'href\')
37             if href and tag == \'a\':
38                 self.urls.append(href)
39 
40     url_seeker = URLSeeker()
41     url_seeker.feed(html)
42     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。

 

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