I don't understand Victor's argument in msg115889.  According to UTF-8 RFC, <http://www.ietf.org/rfc/rfc2279.txt>:

   -  US-ASCII values do not appear otherwise in a UTF-8 encoded
      character stream.  This provides compatibility with file systems
      or other software (e.g. the printf() function in C libraries) that
      parse based on US-ASCII values but are transparent to other
      values.

This means that printf-like formatters should not care whether the format string is in UTF-8, Latin1, or any other ASCII-compatible 8-bit encoding.  (Passing in multibyte encoding pretending to be bytes would of course lead to havoc, but C type system will protect you from that.)

It is also fairly simple to ssnity-check for UTF-8 if necessary, but in case of PyUnicode_FromFormat, the resulting string will be decoded as UTF-8, so all characters in the format string will be checked anyways.

Am I missing something?