I'm not sure about the method names showmsg() and formatmsg().

Maybe: showwarnmsg() and formatwarnmsg()?

First I used warnings.show() and warnings.format(), but "format()" is already the name of a builtin function.

By the way, I would prefer to use underscore between names, like show_msg() or show_warn_msg(). But I don't know if it's ok to use a different naming scheme in an existing module :-/