Code generation architecture question

[Alex_Gaynor]

On Nov 14, 3:04 am, Aaron Brady <castiro... at gmail.com> wrote:
> On Nov 13, 7:16 pm, Alex_Gaynor <alex.gay... at gmail.com> wrote:
>
> > I'm trying to figure out what the best architecture for doing code
> > generation would be.  I have a set of ASTs that define a program, so
> > what should I do to for code generation.  As I see it the 2 choices
> > are to have the ASTs have a generate code method that returns the
> > correct code for themselves, or to have a seperate class that
> > generates code for a given AST.  The problem with the first is that it
> > isn't very flexible if I want to generate code for a new language/
> > platform, as well as the fact that it is very tightly coupled.
> > However the issue with the other approach is that I can't think of a
> > good way to write it without have a ton of if isinstance() calls.  Any
> > thoughts on what the best appraoch would be?
>
> I think you are seeking the Visitor Pattern.
>
> class A:
>   def meth( self, visitor ):
>     visitor.visit_classA( self )
>
> class B:
>   def meth( self, visitor ):
>     visitor.visit_classB( self )
>
> class Visitor:
>   def visit_classA( self, objinst ):
>     #handle classA
>   def visit_classB( self, objinst ):
>     #handle classB
>
> then A().meth( v ) gets around to getting v.visit_classA called.  The
> code is moderately repetitive.  I believe the Transformer class in the
> 'ast' module uses the visitor pattern, possibly called NodeWalker or
> something.

Ah this is great, the Visitor class knows how to transform everything
into code, and each AST knows which method it is on the Visitor,
thanks a ton!