Web scraping with Python

Tools

• python
• html5lib
• ElementTree part of the standard python library

Scraping dmoz.org

Example 1: Pulling the URLs + textual descriptions from a single page

Consider a single page on dmoz, such as:

http://www.dmoz.org/Science/Astronomy/

If you look into the source, we can see the structure around the URLs listed at the bottom of the page:

<ul style="margin-left:0;" class="directory-url">
<li>
<a class="listinglink" href="http://www.absoluteastronomy.com/">Absolute Astronomy</a> 
- Facts and statistical information about planets, moons, constellations, stars, galaxies, and Messier objects.
<div class="flag"><a href="/public/flag?cat=Science%2FAstronomy&amp;url=http%3A%2F%2Fwww.absoluteastronomy.com%2F"><img title="report an issue with this listing" alt="[!]" src="/img/flag.png"></a></div>
</li>

url = sys.argv[1]

f = urllib2.urlopen(url)
src = f.read()
tree = html5lib.parse(src, namespaceHTMLElements=False)

for div in tree.findall(".//ul"):
    if "directory-url" in div.get("class", "").split():
        for li in div.findall("li"):
            for a in li.findall("a"):
                if "listinglink" in a.get("class", "").split():
                    linkurl = a.get("href")
                    linkdescription = a.tail.strip().strip("-").strip()
                    print (linkurl)
                    print ("\t"+linkdescription.encode("utf-8"))
                    print ()

Example 2: Digging into sub / related categories

The pages also contain links to sub- and related categories, if we want to follow these as well, we can...

<div class="dir-1 borN">
    <span><img style="height:2px;float:left;width:100%" src="http://o.aolcdn.com/os/dmoz/img/dividerN.gif"></span>
    <ul class="directory dir-col">
        <li class="">
            <a href="/Science/Anomalies_and_Alternative_Science/Astronomy%2C_Alternative/">Alternative</a>@ 
            <em>(72)</em>
        </li>

from __future__ import print_function
import urllib2, html5lib, sys
from urlparse import urljoin

url = sys.argv[1]

todo = [url]
seen = {}

while len(todo) > 0:
    url, todo = todo[0], todo[1:]
    if url not in seen:
        f = urllib2.urlopen(url)
        print("VISITING", url)
        seen[url] = True
        src = f.read()
        tree = html5lib.parse(src, namespaceHTMLElements=False)

        # Extract links
        print ("LINKS")
        for div in tree.findall(".//ul"):
            if "directory-url" in div.get("class", "").split():
                for li in div.findall("li"):
                    for a in li.findall("a"):
                        if "listinglink" in a.get("class", "").split():
                            linkurl = a.get("href")
                            linkdescription = a.tail.strip().strip("-").strip()
                            print (linkurl)
                            print ("\t"+linkdescription.encode("utf-8"))

        # Follow the related category pages
        print ("RELATED")
        for div in tree.findall(".//ul"):
            if "directory" in div.get("class", "").split():
                for li in div.findall("li"):
                    for a in li.findall("a"):
                        suburl = a.get("href")
                        suburl = urljoin(url, suburl)
                        description = a.text.strip()
                        print (suburl)
                        print ("\t"+description.encode("utf-8"))
                        if suburl not in seen:
                            todo.append(suburl)

        print ()

Example 3: Scraping the links, Crawling to adjacent categories

from __future__ import print_function
import urllib2, html5lib, sys
from urlparse import urljoin


url = sys.argv[1]

todo = [url]
seen = {}
links = {}

while len(todo) > 0:
    url, todo = todo[0], todo[1:]
    if url not in seen:
        f = urllib2.urlopen(url)
        print("VISITING", url)
        seen[url] = True
        src = f.read()
        tree = html5lib.parse(src, namespaceHTMLElements=False)

        # Extract links
        for div in tree.findall(".//ul"):
            if "directory-url" in div.get("class", "").split():
                for li in div.findall("li"):
                    for a in li.findall("a"):
                        if "listinglink" in a.get("class", "").split():
                            linkurl = a.get("href")
                            linkdescription = a.tail.strip().strip("-").strip()
                            if linkurl in links:
                                print ("already seen", linkurl)

                            # Record the link
                            links[linkurl] = linkdescription

        # Follow the related category pages
        for div in tree.findall(".//ul"):
            if "directory" in div.get("class", "").split():
                for li in div.findall("li"):
                    for a in li.findall("a"):
                        suburl = a.get("href")
                        suburl = urljoin(url, suburl)
                        description = a.text.strip()
                        if suburl not in seen:

                            # Add the suburl to the todo list
                            todo.append(suburl)

        print ("links", len(links.keys()))