> Also I think this is often alleviated by using super(). super doesn't help for my use case or not in any way I can see.
> Maybe it is possible to preserve backwards compatibility by making ast_list a subclass of list? Or is it not possible for some reason? There is a layout conflict between AST and list. Caleb Donovick On Fri, Aug 16, 2019 at 5:54 AM Ivan Levkivskyi <levkivs...@gmail.com> wrote: > On one hand I can see how this may cause little inconvenience, but on > other hand this would be a breaking change, so I don't think it is > realistic. > Also I think this is often alleviated by using super(). > > Maybe it is possible to preserve backwards compatibility by making > ast_list a subclass of list? Or is it not possible for some reason? > > -- > Ivan > > > > On Thu, 15 Aug 2019 at 22:32, Caleb Donovick <donov...@cs.stanford.edu> > wrote: > >> When walking an ast it impossible to know the type of an empty list >> without writing down some giant lookup from node types and field names to >> field types. >> >> More concretely it would nice be to able to programatically visit all >> blocks (stmt*) without having to something like: >> >> ``` >> class BlockVisitor(NodeVisitor): >> def visit_If(self, node: If): >> self.visit(node.test) >> self.visit_block(node.body) >> self.visit_block(node.orelse) >> >> def visit_FunctionDef(self, node: FunctionDef): >> for field, value in iter_fields(node): >> if field == 'body': >> self.visit_block(value) >> else: >> # the implementation of generic_visit >> ``` >> Now it turns out that all fields that are lists and are named "body", >> "orelse", or "finalbody" are stmt* and only such fields are stmt*. A rule >> could also be synthesized to identify expr* and so forth but this seems >> incredibly hacky to me. >> >> It would be much cleaner if <type>* were actual nodes in the ast. E.g. >> something like: >> ``` >> class ast_list(AST, MutableSequence[T_co]): ... >> class StmtList(ast_list[stmt]): ... >> class ExprList(ast_list[expr]): ... >> ... >> class FunctionDef(stmt): >> name: identifier >> args: arguments >> body: StmtList >> decorator_list: ExprList >> returns: Optional[expr] >> ``` >> This would not change the behavior or structure in any way other than >> tagging <type>* and allowing <type>* to be visited. >> >> It would potentially break old code which relies on stuff like `if >> isinstance(node.field, list)` e.g. the implementation of generic_visit. >> >> >> Caleb Donovick >> >> _______________________________________________ >> Python-ideas mailing list -- python-ideas@python.org >> To unsubscribe send an email to python-ideas-le...@python.org >> https://mail.python.org/mailman3/lists/python-ideas.python.org/ >> Message archived at >> https://mail.python.org/archives/list/python-ideas@python.org/message/ZHOXQTDSHOERZSGUJXLNJCYLKKQJOYTA/ >> Code of Conduct: http://python.org/psf/codeofconduct/ >> >
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