[Python-3000] Substantial rewrite of PEP 3101

[Nick Coghlan]

Eric V. Smith wrote:
>  >         Formatter.ALLOW_LEADING_UNDERSCORES
>  >         Formatter.CHECK_UNUSED_POSITIONAL
>  >         Formatter.CHECK_UNUSED_NAME

> I'm not sure I'm wild about these flags which would have to be or'd
> together, as opposed to discrete parameters.  I realize have a single
> flag field is likely more extensible, but my impression of the
> standard library is a move away from bitfield flags.  Perhaps that's
> only in my own mind, though!
> 
> Also, why put this in the base class at all?  These could all be
> implemented in a derived class (or classes), which would leave the
> base class state-free and therefore without a constructor.

I think the dict/defaultdict cooperative implementation based on the 
__missing__ method is a good guide to follow here. Instead of having 
flags to the constructor, instead define methods that the base class 
invokes to deal with the relevant checks - subclasses can then override 
them as they see fit.

A couple of possible method signatures:

   def allowed_name(self, name):
       "Return True if name is allowed, False otherwise"
       # default implementation return False if name starts with '_'

   def allow_unused(self, unused_args, unused_kwds):
       "Return True if unused args/names are allowed, False otherwise"
       # default implementation always returns True

Subclasses can then either return False to get a standard 'disallowed' 
exception, or else raise their own exception explicitly.

A few common alternate implementations of the latter method would be:

   def allow_unused(self, unused_args, unused_kwds):
       # All positional arguments must be used
       return not unused_args

   def allow_unused(self, unused_args, unused_kwds):
       # All keyword arguments must be used
       return not unused_kwds

   def allow_unused(self, unused_args, unused_kwds):
       # All arguments must be used
       return not unused_args and not unused_kwds


Cheers,
Nick.


-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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