[Python-3000] string.Formatter class

[Eric Smith]

Ron Adam wrote:
> 
> 
> Eric Smith wrote:
>> 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).
> 
> Cool.  I looked at other possible ways, but this seemed to be the 
> easiest to live with.  The alternative is to use an attributes to pass 
> and hold values, but sense the Formatter class isn't a data class, that 
> doesn't seem appropriate.

I agree that attributes seem like the wrong way to go about it.  It also 
makes recursively calling the formatter impossible without saving state.

I'm still planning on making this change tonight.

>> 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.
> 
> How?  From what I can see an exception would be raised in the get_value 
> method.
> 
> When would I ever want to get a None for args_used?

I meant arg_used.

You're right.  I was confusing get_field with get_value.  (Surely we can 
pick better names!)

Eric.