[Python-Dev] Re: PEP 292, Simpler String Substitutions

[Christian Tismer]

Paul Prescod wrote:
> Christian Tismer wrote:
> 
>>...
>>
>>Are you sure you got what I meant?
>>I want to compile the variable references away at compile
>>time, resulting in an ordinary format string.
>>This string is wraped by the runtime _(), and
>>the result is then interpolated with a dict.
> 
> 
> How can that be?
> 
> Original expression:
> 
> _($"$foo")
> 
> Expands to:
> 
> _("%(x1)s"%{"x1": foo})
> 
> Standard Python order of operations will do the %-interpolation before
> the method call! You say that it could instead be 
> 
> _("%(x1)s")%{"x1": foo}
> 
> But how would Python know to do that? "_" is just another function.
> There is nothing magical about it. What if the function was instead
> re.compile? In that case I would want to do the interpolation *before*
> the compilation, not after!
> 
> Are you saying that the "_" function should be made special and
> recognized by the compiler?

As you say it, it looks a little as if something special
would be needed, right.
I have no concrete idea.
Somehow I'd want to express that a function is
applied after compile time substitution, but before
runtime interpolation.

Here a simple idea, while not very nice, but it could work:

Assume a "$" prefix, which does the interpolation in the
way you said.
Assume further a "%" prefix, which does it only halfway,
returning a tuple: (modified string, dict).
This tuple would be passed to _(),
and it is _()'s decision to work this way:

def _(s):
     if type(s) == type(()):
         s, args = s
     else:
         args = None

#... processing s ...
     if args:
         return s % args
     else:
         return s

But this is a minor issue, I just wanted to tell what
I think should happen, without giving an exact
solution.

cheers - chris

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