[Tutor] Re: Factory classes (etc)

[Scot W. Stevenson]

Hello Magnus, 

Thank you for the long answer!

> Your spray can example is neither an "Abstract Factory" nor a "Factory
> Method" I'm afraid. Both the "Abstract Factory" and "Factory Method"
> patterns return instances of _different_classes_ depending on the
> situation. 

I knew it was too good to be true =8). 

> # Factory function for symmetrical shapes
> def getSymmetricalShape(x, y, size, corners=0, angle=0):
>      assert type(corners) == type(1) and (corners == 0 or corners > 1)
>      if corners == 0:
>          return Circle(x, y, size)
>      elif corners == 2:
>          return Line(x, y, size, angle)
>      elif corners == 3:
>          return Triangle(x, y, size, angle)
>      elif corners == 4:
>          return Square(x, y, size, angle)
>      else:
>          return RegularPolygon(x, y, size, sides, angle)

Ah, now I understand. Just as an aside, if I were to really programm 
something like this (which is certainly the best way to understand the 
principle), would I want to use a if-elif-else construct like above, or 
rather a dictionary such as:

shapes = {0: Circle, 2: Line, 3: Triangle, [...]}

and then do a 

shapes[corners](x, y, size, angle)

which would assume that Circle doesn't go balistic if it is suddenly handed 
an angle. Again, I'm glad you did it this way here, because it is easier 
to understand. 

> This is a bit strange, since the Pattern
> movement (in software design) started among SmallTalkers, and SmallTalk
> is dynamically typed--I don't know enough SmallTalk to explain this. 

Does anybody actually _use_ SmallTalk for production code, or is it one of 
those teaching languages that revolutionized various concepts but never 
made it in the market (sort of like Multics [sp] vs. Unix, or the League 
of Nations vs. the United Nations, or for that matter, "Buffy" the Film 
vs. "Buffy" the TV series)?

> I'm not saying that the ideas in GoF are overly complex, but I think
> it would have been a much thinner book if the Gang of Four had used
> Python...

Are there any common patterns that are missing by today's standards? I'm 
thinking about the Borg "non-pattern" described in the "Python Cookbook" - 
I can't believe that seven years later, the GoF is still state of the 
art...

Thanks again,
Y, Scot


-- 
  Scot W. Stevenson wrote me on Thursday, 5. Sep 2002 in Zepernick, Germany   
       on his happy little Linux system that has been up for 1903 hours       
        and has a CPU that is falling asleep at a system load of 0.08.