It's easy enough to implement. Although the 'all-or-nothing' aspect is a
little tough, I'll have to give it some thought.

Maybe the best way to do this is to first create a string.Formatter
subclass that implements it. I'll play with it and see what I come up with.

From the description, I'm presuming we'd want:
'{d}{s}{f}'.format(3, 'foo', 3.14)
to work.