> On 16 Aug 2019, at 21:44, Caleb Donovick <donov...@cs.stanford.edu> wrote: > > > This part could be solved without making the lists AST nodes at all. Just > > use new bare subclasses of list for each of the kinds of lists, so a field > > in a node never has an empty list, it has an empty ExprList or StmtList or > > whatever. If that’s sufficient for your needs, that seems pretty easy, and > > I don’t think it would have any compatibility problems. > This would probably be sufficient. It would make it pretty easy to write > replacement versions of what I want from ast.py with the behavior I want. > > > Also, while a I can see a use for StmtList (because every StmtList is a > > block or a module body, so they all have quite a bit in common), where > > would you use ExprList? > You are right I only really care about StmtList. Just seemed like a strange > asymmetry to have StmtList but not ExprList ect... > > > But I don’t think there’s any actual behavior in AST that you need (except > > maybe for looking like a structseq, with _fields, but you can fake that > > with a plain old class a la namedtuple or dataclass, as long as there’s no > > C code that relies on being able to iterate the structseq fields). > I really don't care about the SmttList being a structseq or at least I > don't think I do. I am pretty unfamiliar with the C bits of CPython. > > Both your suggestions seem completely sufficient to me. > > ---- > > > Just to make sure we're on the same page: what are you using the ast > > module for? > I am building DSLs and need to perform AST analysis / rewriting. I commonly > perform block level analysis but it gets pretty verbose because of all the > different places stmt* can be. > I am unfamiliar with parso but it looks like it has some nice convenience > functions. It probably won't be useful for me though because I need to be > able to exec the AST.
You can dump the AST back to a string with Parso. In fact it's built in and way better because it keeps the formatting and comments! > Further, it is highly desirable for me to be able to turn the AST back into a > string (as astor allows) so that I can generate reasonable error messages and > debug. Then you really should look at parso! > >> On Fri, Aug 16, 2019 at 1:24 PM Anders Hovmöller <bo...@killingar.net> wrote: >> Just to make sure we're on the same page: what are you using the ast module >> for? >> >> Maybe moving to another lib like parso actually helps your real problem >> more... >> >> > On 15 Aug 2019, at 22:02, 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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