Question

I’m reading Dive into Python, especially trying to understand the examples, and I had some questions about list-urls.py.

In the last line it compiles the lists of urls from “parser.urls”. Where is this data coming from? I don’t see a urls method in URLLister or SGMLparser.

Also a method start_a was created but never used. What is this?

Link to full code, below is a condensed version http://pastebin.com/EbB4micK

#!/usr/bin/python
"""Extract list of URLs in a web page"""

from sgmllib import SGMLParser
import sys

class URLLister(SGMLParser):
    def reset(self):
            SGMLParser.reset(self)
            self.urls = []

    def start_a(self, attrs):
            href = [v for k, v in attrs if k=='href']
            if href:
                    self.urls.extend(href)

if __name__ == "__main__":

    link = sys.argv[1]
    try:
            usock = urllib.urlopen(link)
            parser = URLLister()
            parser.feed(usock.read())
            parser.close()
            usock.close()
            for url in parser.urls: print url

Answer 1

It’s an attribute, bound and mutated within the methods.

        self.urls = []

…

                self.urls.extend(href)

start_a() is part of the protocol of SGMLParser, of which URLLister is a descendant.

Apart from overriding or extending the methods listed above, derived classes may also define methods of the following form to define processing of specific tags. Tag names in the input stream are case independent; the tag occurring in method names must be in lower case: