> b'%c' is still raising a TypeError.  The error message is fine ("%c requires an integer in range(256) or a single byte") but it should be an OverflowError for backwards compatibility.

I don't understand why you care so much on the exact exception. It doesn't look right to me to rely on the *exact* exception raised by "%c" % arg. It's an obvious bug in the application.

Sometimes, you may want to be extra safe and catch exception while formating a message. The logging module does this. But the logging doesn't care of the exact exception, it uses a generic "except Except:" in StreamHandler.emit():

    def emit(self, record):
        try:
            msg = self.format(record)
            stream = self.stream
            stream.write(msg)
            stream.write(self.terminator)
            self.flush()
        except Exception:
            self.handleError(record)

IMO b"%c" % int must raise ValueError, not OverflowError, if the value is not in the range 0..255.