Yeah, I was thinking about this a bit more and realised that I'd rejected the "quote everything by default" approach before I had the idea of providing a custom conversion specifier to disable the implicit string conversion and quoting.

So perhaps a better alternative would be:

  default - str + shlex.quote
  !u - unquoted (i.e. normal str.format default behaviour)

When you have a concise way to explicitly bypass it, making the default behaviour as safe as possible seems like a good way to go.