calling subclass constructor question

[Steven Bethard]

In Han Kang wrote:
> So each of the sub classes plots a different type of graph.  The 
> superclass defines methods that are the same for all the plots.  I want 
> to be able to pick some points and be able to generate a more plots.  
> What I was wondering if I could define in a method in the superclass of 
> an object the ability to make a brand new subclass (new plot).  So 
> basically, one plot makes another plot, but it'd be nice if i could put 
> all the code in the superclass.

Still not sure I understand you.  How are you determining which new type 
of plot to create?  If I'm an instance of class PlotA, and the user asks 
me to create create a new Plot, is it always another instance of class 
PlotA, or could it be an instance of another class?

If the former, you can use a classmethod, e.g.:

class Plot(object):
     @classmethod
     def new(cls, ...):
         result = cls(...)
         # maybe modify result some more
         return result

If the latter, this doesn't sound like something that should be a method 
of an instance.  It sounds more like a factory function.  Why not just 
make it a module-global function, e.g.:

def newPlot(...):
     # determine which kind of Plot object to create
     ...
     if ...:
         return PlotA(...)
     ...

A method signature of the method you want to write would help a lot...

STeVe