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

["Martin v. Löwis"]

>>> There's a lot of code already out there (in the standard library and
>>> other places) that uses %-style formatting, when in Python 3.0 we
>>> should be encouraging {}-style formatting. 
>>
>> I don't agree that we should do that. I see nothing wrong with using
>> % substitution.
> 
> It's a maintenance burden.

Well - that's the cost of keeping it in the language. It's not a problem
with using it while it *is* in the language.

So if a decision was made to eventually remove % formatting, it would
be reasonable to start migrating code to PEP 3101. However, no such
decision has been made (and hopefully won't be throughout 3.x), so
as the mechanism *is* available, there is no need to start changing
existing code (except the for actual issue Steven discusses, namely
libraries that expect strings in % template form).

> I'd note that PEP 3101 calls str.format() a replacement for
> %-formatting, not an alternate mechanism to achieve the same end.

I think this is a mis-wording; the intent of the PEP apparently is
to propose this mechanism as an option, not as an actual replacement.
This becomes clear when reading the "Backwards Compatibility" section:

# Backwards compatibility can be maintained by leaving the existing
# mechanisms in place.  The new system does not collide with any of
# the method names of the existing string formatting techniques, so
# both systems can co-exist until it comes time to deprecate the
# older system.

Regards,
Martin