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

Nick Coghlan ncoghlan at gmail.com
Sat Oct 3 11:37:00 CEST 2009

Bugbee, Larry wrote:

So until I see a significant benefit, my vote is not remove %-formatting. Make both available and if {} is to win, it will.

Percent formatting isn't going anywhere (certainly not for the next decade or so). However, its well documented limitations (especially the lack of extensibility) and error-prone syntax (handling of dicts and tuples, forgetting trailing type codes in namespace formatting, operator precedence issues) are what led directly to PEP 3101 and the introduction of brace formatting.

For the basic cases that percent formatting actually covers, it does a reasonable job and is often more concise and currently executes faster than the brace formatted equivalent (this was especially the case before support for implicit positional argument numbering was added to brace formatting).

The advantages of brace formatting don't really start to show up until you actually start using the new features that the approach provides, such as formatting for non-builtin types (e.g. Decimal and datetime objects), referring to argument subscripts and attributes in field definitions and the optional explicit numbering of references to positional arguments (allowing the order of interpolation to be changed in the format string without having to change the argument order in all uses of that format string).

Also, don't forget that the percent formatting code has been around for more than a decade and a half and has been optimised over that time. The brace formatting code, on the other hand, is relatively new and probably still offers plenty of opportunities for optimisation (although the additionally flexibility in the brace formatting approach means that it is unlikely to ever catch up completely to the raw speed of percent formatting).

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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