[Tutor] Clash of the Titans and Mundane Matters

[Sean Perry]

Michael Powe wrote:
> Clash of the Titans
>

snip constructor discussions

Pilgrim is pedantically correct but Alan's comment matches how most of 
us think about it.

> 
> ----------------
> Mundane Matters
> 
> I'm having a hard time with classes in python, but it's coming
> slowly.  One thing that I think is generally difficult is to parse a
> task into "objects."  Here's an example:  in Java, I wrote an
> application to track my travelling expenses (I'm a consultant; this
> tracking of expenses is the itch I am constantly scratching.  ;-)
> I've also written this application in a perl/CGI web application as
> well.)  It's easy to see the outline of this task:  create an abstract
> class for expense and then extend it for the particular types of
> expenses -- travel, food, transportation, lodging and so forth.  In
> python, I guess I'd create a class and then "subclass" it.

while that is a valid approach, it is not how most of us would do it. By 
subclassing you have to edit the code every time a new expense type is 
added. Ever used MS Money or Quicken? Imagine if the type of each item 
was a subclass. Use a string.

> 
> A similar problem occurs with my HTML-parsing routine that I brought
> to the list recently.  Use of HTMLParser was suggested.  I've looked
> into this and usage means subclassing HTMLParser in order to implement
> the methods in the way that will accomplish my task.  Conceptually,
> I'm having a hard time with the "object" here.  (The fairly poor
> documentation for HTMLParser doesn't help.)  Apparently, I'm creating
> a "parser" object and feeding it data.  At least, that's the closest I
> can get to understanding this process.  How I'm actually feeding data
> to the "parser" object and retrieving the results are matters open to
> discussion.  I'll be working on that when I get another chance.
>

This counts the tags in a html file piped in on stdin.

#!/usr/bin/python 


import sys, HTMLParser

class TagCounter(HTMLParser.HTMLParser):
     def __init__(self):
         HTMLParser.HTMLParser.__init__(self)
         self.tags = {}

     def handle_starttag(self, tag, attrs):
         self.tags[tag] = self.tags.setdefault(tag, 0) + 1

if __name__ == '__main__':
     counter = TagCounter()
     for line in sys.stdin.xreadlines():
         counter.feed(line)
     counter.close()
     print counter.tags

> Finally, in terms of "understanding python," the question I keep
> coming up against is:  why do we have both functions and methods?
> What is the rationale for making join() a string method and a os.path
> function?
>

a method is a function bound to a class. Nothing super special.