[Python-Dev] Recent experience with the _ast module

[Brett Cannon]

On 2/16/07, Collin Winter <collinw at gmail.com> wrote:
> On 2/14/07, "Martin v. Löwis" <martin at v.loewis.de> wrote:
> > Collin Winter schrieb:
> > > What's inconsistent about it? That classes are being used for the
> > > _ast.{Add,Sub,Mult,etc} names?
> >
> > Exactly. These aren't names - they are nodes in the tree. All nodes
> > are instances of _ast.AST.
> >
> > > I don't see the need for both _ast.Add and _ast.Add.singleton or
> > > _ast.add or however else it might be spelled. I'd be perfectly happy
> > > doing something like "_ast.Add = object()" (when initializing the _ast
> > > module), so long as I can write "node.op is _ast.Add", "node.op ==
> > > _ast.Add", or something equally brief and to-the-point.
> >
> > Would you like to do the same for Pass, Break, Continue, and Ellipsis?
> >
> > They are also "just names". If you make _ast.Add the entry in the
> > tree itself, why not _ast.Break? Or, if you have a way to deal with
> > _ast.Break, why can't the same way work for _ast.Add?
>
> But Pass, Break, Continue and Ellipsis aren't in the same category as
> Add, Mult, Div, etc.. The former stand alone, while the only time
> you'll ever encounter Add, Mult, Mod and co. is in the context of a
> BoolOp, BinOp or whatever. I could of course add separate visitor
> methods for each op type to my code generator, but that's a lot of
> boilerplate code for very little gain. I could use a dispatch table in
> visitBoolOp, and dispatch like "dispatch[type(node.op)]", but why? In
> this scenario, node.op doesn't convey any information other than its
> type. Add, Mult, etc are just values in an enumeration, so why not
> treat them that way and be able to write "dispatch[node.op]".
>
> So: what if _ast.Add and co. were singleton instances of _ast.AST or
> even of a subclass of AST, like _ast.Op? That way, we keep the
> consistency that all AST nodes are instances of _ast.AST and I get the
> nice property I'm looking for, ie, "node.op is _ast.Mult".
>

If you look back in the history of this thread I think Martin
suggested this: "I'd rather prefer
if the singletons where exposed under a name, e.g. _ast.Add.singleton,
or _ast.add (if that doesn't cause conflicts)."

Since the both of you have proposed this I think this is the best way
to move forward.

-Brett