If you try this sort of inheritance, I'd recommend writing down the formal grammar before you start writing classes. Don't try to define the grammar through the inheritance hierarchy; it's too easy to accidentally build a hierarchy that can't be translated into a single-pass-parsable grammar...
I usually skip the inheritance and make everything an instance of the same class, e.g. class ASTNode(object): ... class Stmt(ASTNode): ... class Expr(ASTNode): ... class UnaryExpr(ASTNode): ... class BinaryExpr(ASTNode): ... or you could dynamically generate classes with inheritance based on a grammar definition -- http://mail.python.org/mailman/listinfo/python-list
