[Python-3000] string.Formatter class

[Eric Smith]

Ron Adam wrote:
>> get_field(field_name, args, kwargs, used_args)
>> Given a field_name as returned by parse, convert it to an object to be 
>> formatted.  The default version takes strings of the form defined in 
>> the PEP, such as "0[name]" or "label.title".  It records which args 
>> have been used in used_args.  args and kwargs are as passed in to 
>> vformat.
> 
> Rather than pass the used_args set out and have it modified in a 
> different methods, I think it would be better to pass the arg_used back 
> along with the object.  That keeps all the code that is involved in 
> checking used args is in one method.  The arg_used value may be useful 
> in other ways as well.
> 
>      obj, arg_used = self.get_field(field_name, args, kwargs)
>      used_args.add(arg_used)

I'm really not wild about either solution, but I suppose yours is less 
objectionable than mine.  I'll check this change in tonight (before the 
deadline).

I think you'd have to say:

if args_used is not None:
    used_args.add(args_used)

as it's possible that the field was not derived from the args or kwargs.

> I wonder if this is splitting things up a bit too finely?  If the format 
> function takes a conversion argument, it makes it possible to do 
> everything by overriding format_field.
> 
>     def format_field(self, value, format_spec, conversion):
>         return format(value, format_spec, conversion)
> 
> 
> Adding this to Talins suggestion, the signature of format could be...
> 
>     format(value, format_spec="", conversion="")

But this conflates conversions with formatting, which the PEP takes 
pains not to do.  I'd rather leave them separate, but I'll let Talin 
make the call.

Eric.