[Python-Dev] Re: Alternative ImplementationforPEP292:SimpleString Substitutions

[Barry Warsaw]

On Fri, 2004-09-10 at 18:22, Raymond Hettinger wrote:

> > My only problem with that is the interference that the 'mapping'
> > argument presents.  IOW, kwds can't contain 'mapping'. 
> 
> To support a case where both a mapping and keywords are present, perhaps
> an auxiliary class could simplify matters:
> 
>    def substitute(self, mapping=None, **kwds):
>        if mapping is None:
>            mapping = kwds
>        elif kwds:
>            mapping = _altmap(kwds, mapping)
>         . . .
> 
> class _altmap:
>     def __init__(self, primary, secondary):
>         self.primary = primary
>         self.secondary = secondary
>     def __getitem__(self, key):
>         try:
>             return self.primary[key]
>         except KeyError:
>             return self.secondary[key]

> This matches the way keywords are used with the dict().

This isn't exactly what I was concerned about, but I agree that it's a
worthwhile approach.  (I'm going to accept your patch and check it in,
with slight modifications.)

What I was worried about was if you providing 'mapping' positionally,
and kwds contained a 'mapping' key, you'll get a TypeError.  I'm going
to change the positional argument to '__mapping' so collisions of that
kind are less likely, and will document it in libstring.tex.

-Barry

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