[Python-Dev] Recent experience with the _ast module

[Brett Cannon]

On 2/12/07, Collin Winter <collinw at gmail.com> wrote:
> I've been working this past week at converting the stdlib's compiler
> package to use 2.5's new _ast module instead of the package's own AST.
> Overall, _ast was a joy to work with, save for the following nitpicks:
>
> 1) There are times when the _fields attribute on some AST nodes is
> None; if this was done to indicate that a given node has no
> AST-related attributes, it would be much easier if _fields was [] in
> this case. As it is, I had to special-case "node._fields is None" in
> the visitor so that I don't accidentally iterate over it.
>

That makes total sense to me.

> 2) {BinOp,AugAssign,BoolOp,etc}.op is an instance of, eg, Add, Sub,
> Mult, Mod, meaning you have to dispatch based on tests like
> "isinstance(node.op, x)" or "type(node.op) is x". I would much, much
> prefer to spell this "node.op is x", ie, use "node.op = Add" rather
> than the current "node.op = Add()" when constructing the nodes.
>

I can't think of a reason, off the top of my head, why they can't be
singletons.  It actually makes sense since they are basically an
enumeration value in the AST grammar.

> 3) I'd like there to be an Else() node for If.orelse, While.orelse,
> etc. My motivation is that I need the lineno and col_offset values of
> the "else" statement for a code-coverage utility; as it is, I have to
> find the last lineno of the if/while/etc suite and the first lineno of
> the "else" suite and then search between those two for the "else"
> statement.
>

Ouch.  I don't know how much work it would be to make this change, but
it seems reasonable to want.

> Thoughts?
>

They all sound reasonable.  And it's nice to have a wanted feature be
found from actual use.  =)

-Brett