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

[Christian Tismer]

Extended proposal at the end:

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?

My idea has evolved into the following:
Consider an interpolating object with the following
properties (sketched by a class here):

class Interpol:
     def __init__(self, fmt, dic):
         self.fmt = fmt
         self.dic = dic
     def __repr__(self):
         return self.fmt % self.dic

Original expression:

_($"$foo")

Expands at compile time to:

_( Interpol("%(x1)s", {"x1": foo}) )

Having said that, it is now up to the function _()
to test whether its argument is an Interpol or not.
It can do something like that:

def _(arg):
     ...
     if type(arg) is Interpol:
         return _(arg.fmt) % arg.dic

# or, maybe cleaner, leaving the formatting action
# to the Interpol class:

def _(arg):
     ...
     if isinstance(arg, Interpol):
         return arg.__class__(_(arg.fmt), arg.dic)

# which then in turn will return the final string,
# if it is interrogated via str or repr.

ciao - chris

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