New Python 3.0 string formatting - really necessary?

[Aaron Brady]

On Dec 20, 8:49 pm, MRAB <goo... at mrabarnett.plus.com> wrote:
> Aaron Brady wrote:
> > On Dec 20, 7:38 pm, Steven D'Aprano <st... at REMOVE-THIS-
> > cybersource.com.au> wrote:
> >> Instead of just whinging, how about making a suggestion to fix it? Go on,
> >> sit down for an hour or ten and try to work out how a BINARY OPERATOR
> >> like % (that means it can only take TWO arguments) can deal with an
> >> arbitrary number of arguments, *without* having any special cases.
>
> >> Go on. Take your time. I'll be waiting.
>
> > Hi, not to take sides, but, there is a possibility.
>
> > This behavior is currently legal:
>
> >>>> "%i %%i" % 0 % 1
> > '0 1'
>
> > So, just extend it.  (Unproduced.)
>
> >>>> "%i %i" % 0 % 1
> > '0 1'
> >>>> "%r %i" % (2, 3, 4) % 1
> > '(2, 3, 4) 1'
> >>>> "%r %i" % (2, 3, 4)
> > '(2, 3, 4) %i'
>
> > Which is quite clever and way ahead of its (posessive) time.
>
> A couple of problems:
>
> 1. How do you handle a literal '%'? If you just double up then you'll
> need to fix the string after all your substitutions.
>
> 2. What if a substitution introduces a '%'?
>
> I suppose a possible solution would be to introduce a special format
> string, including a literal, eg:
>
>      f"%r %i" % (2, 3, 4) % 1
>
> and then convert the result to a true string:
>
>      print(str(f"%r %i" % (2, 3, 4) % 1))
>
> (although print() would call __str__ anyway).
>
> The format string would track where the last substitution occurred.
>
> Hmm... I think I'll just learn the new method. :-)

Now that I'm fighting 'r's war for him/her...

Um, here's one possibility.  On the first interpolation, flags are
noted and stored apart from subsequent interpolations.  Then, use a
sentinel to terminate the interpolation.  (Unproduced.)

>>> "%r %i" % ( 2, 3 ) % 0
'(2, 3) 0'
>>> "%% %r" % ( 2, 3 ) % str.interp_end
'% (2, 3)'
>>> "%sss%i" % "%d" % 0
'%dss0'

The first %s is replaced with %d, but doesn't hijack the '0'.  If you
want to interpolate the %d, use the sentinel.  The sentinel is what
causes '%%' to be handled.

>>> "%sss%i" % "%d" % 0 % 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting
>>> "%sss%i" % "%d" % 0 % str.interp_end % 1
'1ss0'

Treating tuples as a special case appears to be the simpler solution,
but this, 'chaining', to adopt the term, is still feasible.