[Python-Dev] transitioning from % to {} formatting

[Christian Tanzer]

Eric Smith wrote at Thu, 08 Oct 2009 10:55:21 -0400:

> >>> BTW I sent Eric a private mail re. the "0o" versus "0" issue, to see if it was
> >>> worth raising an enhancement request on the bug tracker using "O" to generate
> >>> compatible rendering for octals.
> >> I didn't get your message, could you resend?.
> >>
> >> I was thinking the same thing, but it seems like a transition step. I'd
> >> rather not keep such backward compatibility hacks (if you will) around
> >> for the long haul.  How about a flag (maybe '*') at the start of the
> >> format specification which says "operate in backward compatibility
> >> mode"? We could document it as being only useful for the % to {}
> >> translator, and promise to remove it at some point in the future. Either
> >> actually deprecate it or just promise to deprecate it in the future.
> >
> > That doesn't seem very useful to me. IIUC, the point of the translator
> > is to allow porting of the millions of existing %-formating operations
> > to the new-style .format.
> >
> > If the result of that is deprecated or removed a few years from now,
> > all maintainers of long existing code have exactly the same problem.
>
> I was thinking of it as a transition step until all application code
> switched to {} formatting. In which case the application has to deal
> with it.

You lost me here.

All that talk of deprecating %-formatting makes me really nervous.
%-formatting is pervasive in all existing Python code.

Without an automatic translator that is 100% accurate, porting all
that code to {}-formatting is not possible. Heck, it's not even
possible to grep for all instances of %-formatting.

How do you suppose that maintainers could ever do the transition from
%- to {}-formatting manually?

> > IMHO, either the translation is done once and gives identical output or
> > it isn't worth doing at all.
>
> I disagree. I doubt even 0.001% of all format strings involve octal
> formatting. Is it really worth not providing a transition path if it
> can't cover this case?

If %-formatting is first deprecated then removed from Python and there
is no automatic transition path that effectively means that existing
code using %-formatting is forced to stay at whatever Python version
was the last one supporting %-formatting.

I surely hope nobody is seriously considering such a scenario. Perl 6
seems harmless in comparison.

--
Christian Tanzer                                    http://www.c-tanzer.at/