Re: [Tutor] Larger program organization

[Kent Johnson]

Terry Carroll wrote:
On Fri, 11 Feb 2005, Bob Gailer wrote:

Whenever you find yourself writing an if statement ask whether this
would be better handled by subclasses. Whenever you find yourself about
to write a global statement, consider making the variables properties of
a class.

Bob -- 

Brian already asked for an explanation of your first statement, and I 
found the ensuing discussion very instructive.

Can you explain the second?  As an aesthetic point, I hate globals, and 
I'd love a discussion with some examples of using class variables as a way 
of avoiding this.
Global variables are one way to make state persist across function calls. Here's a toy example that 
might give you the idea. Suppose you want to write a function that keeps a running total. You could 
do something like this (not recommended!):

total = 0
def addToTotal(inc):
  global total
  total += inc
Every time you call addToTotal, total is incremented.
This is already a poor design.
- There is no encapsulation - to increment the total, you call a function, to view the total you 
look at the global variable.
- There is no API - to reset the total you would have to set the global variable.

You could try to fix this by adding more functions:
def printTotal():
  print 'Total is', total
def resetTotal():
  global total
  total = 0
That's a little better, maybe. But there are other problems:
- You can only have one total. What if you want two different totals?
- Your global namespace has extra names - total, addToTotal, etc.
For short scripts this structure can work, but for larger projects it gets unwieldy. OOP to the 
rescue! How about a Total class?

class Total:
  def __init__(self):
    self.reset()
  def add(self, inc):
    self.total += inc
  def print(self):
    print 'Total is', self.total
  def reset(self):
    self.total = 0
You can use this like this:
t = Total()
t.inc(5)
t.print()
t.reset()
Now everything is wrapped up in a nice neat package. There is a clear, consistent API, no namespace 
pollution, and you have a reusable object.

You might also be interested in this essay: 
http://www.pycs.net/users/0000323/stories/15.html
Kent
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