Code generation architecture question

[Alex_Gaynor]

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?