Partial string formatting?

[Gonçalo Rodrigues]

Hi,

Is there a simple way to do partial string formatting short of using
RE's, e.g

<string> % <mapping>

where if a key is not present in <mapping> it just leaves it untouched?

I thought of subclassing dict to hand out a default item, which it would
just be the requested key wrapped in %()s. Of course this only works if
the format code is s but not with other format codes or even more
complicated options.

Thanks in advance,
G. Rodrigues