[Python-Dev] transitioning from % to {} formatting (original) (raw)

James Y Knight foom at fuhm.net
Fri Oct 2 00:03:47 CEST 2009

On Oct 1, 2009, at 5:54 PM, Nick Coghlan wrote:

I believe classes like fmtbraces/fmtdollar/fmtpercent will be part of a solution, but they aren't a complete solution on their own. (Naming the three major string formatting techniques by the key symbols involved is a really good idea though)

1. It's easy to inadvertently convert them back to normal strings. If a formatting API even calls "str" on the format string then we end up with a problem (and switching to containment instead of inheritance doesn't really help, since all objects implement str).

Using containment instead of inheritance makes sure none of the
other operations people do on strings will appear to work, at least
(substring, contains, etc). I bet explicitly calling str() on a format
string is even more rare than attempting to do those things.

2. They don't help with APIs that expect a percent-formatted string and do more with it than just pass it to str.mod (e.g. inspecting it for particular values such as '%(asctime)s')

True, but I don't think there's many such cases in the first place,
and such places can be fixed to not do that as they're found.

Until they are fixed, fmt_braces will loudly fail when used with that
API (assuming fmt_braces is not a subclass of str).

James

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