[Python-Dev] PEP 292, Simpler String Substitutions

[Oren Tirosh]

On Wed, Jun 19, 2002 at 02:45:46AM -0400, Raymond Hettinger wrote:
> > Handling Missing Keys
> >
> >     What should happen when one of the substitution keys is missing
> >     from the mapping (or the locals/globals namespace if no argument
> >     is given)?  There are two possibilities:
> >
> >     - We can simply allow the exception (likely a NameError or
> >       KeyError) to propagate.
> >
> >     - We can return the original substitution placeholder unchanged.
> 
> And/Or,
>        - Leave placeholder unchanged unless default argument supplied:
>                   mystr.sub(mydict, undefined='***')  # Fill unknowns with
> stars
> And/Or,
>        - Raise an exception only if specified:
>                   mystr.sub(mydict, undefined=NameError)
> And/Or
>         - Return a count of the number of missed substitutions:
>                   nummisses = mystr.sub(mydict)
> 
> >     BDFL proto-pronouncement: It should always raise a NameError when
> >     the key is missing.  There may not be sufficient use case for soft
> >     failures in the no-argument version.
> 
> I had written a minature mail-merge program and learned that the NameError
> approach is a PITA.  It makes sense if the mapping is defined inside the
> program;

Exceptions are *supposed* to be a PITA in order to make sure they are hard 
to ignore.

+1 on optional argument for default value. 
-1 on not raising exception for missing name. 

I think the best approach might be to raise NameError exception *unless* a 
default argument is passed.

The number of misses cannot be returned by this method - it returns the new
string. 

	Oren