How to set a class inheritance at instance creation?

[glomde]

On 29 Maj, 22:45, Steve Holden <s... at holdenweb.com> wrote:
> glomde wrote:
> > Hi I wonder if you can set what subclass a class should
> > have at instance creation.
>
> > The problem is that I have something like:
>
> > class CoreLang():
> >     def AssignVar(self, var, value):
> >          pass
>
> > class Lang1(CoreLang):
> >      def AssignVar(self, var, value):
> >           return var, "=", value
>
> > class Lang2(CoreLang):
> >      def AssignVar(self, var, value):
> >           return var, "<=", value
>
> > class WriteStruct():
> >      def Generate(self, vars):
> >         for var in vars:
> >              print self.AssignVar()
>
> > The problem is that I want  WriteStruct to sometimes be a subclass of
> > Lang1 and sometimes
> > of Lang2.
> > In the above example I could but the Generate Method in CoreLang. But
> > in my real
> > example I also want to able to subclass WriteStruct to be able to easy
> > customize WriteStruct.
> > Which I wouldnt be able to do if it was a method in CoreLang.
>
> > So in code I would like to write something like:
>
> > WriteStruct(Lang1).Generate(vars)
>
> > Even better would be that if I in the Lang1 class could
> > just do WriteStruct().Generate(vars) and Lang1 class would
> > magically make WriteStruct a subclass of itself.
>
> You should rethink your program design.
>
> It seems that when you create a WriteStruct you should really be passing
> its __init__() method the class that you want it to be a "subclass" of,
> creating an instance of that class, and then using generic delegation to
> that subclass (using a modified __getattr__()) to handle methods that
> aren't found in the WriteStruct.

This is what I am going to do. For some reason I got stuck to think
I needed to solve it with inheritance.

Didnt think of the possibility to modify getattr to make
the delegation be much nicer. Thanks for the tip


>
> I can see there are circumstances in which this might not work, but I
> believe your current ugly intentions reveal a design smell that you
> really need to get rid of if you want a clean program.
>
> regards
>   Steve
> --
> Steve Holden        +1 571 484 6266   +1 800 494 3119
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