code: def format(self, record):
          
          # NOTE(sdague): default the fancier formating params
          # to an empty string so we don't throw an exception if
          # they get used
          for key in ('instance', 'color'):
              if key not in record.__dict__:
                  record.__dict__[key] = ''
  
          if record.__dict__.get('request_id', None):
              self._fmt = CONF.logging_context_format_string
          else:
              self._fmt = CONF.logging_default_format_string
  
          if (record.levelno == logging.DEBUG and
                  CONF.logging_debug_format_suffix):
              self._fmt += " " + CONF.logging_debug_format_suffix
  
          # Cache this on the record, Logger will respect our formated copy
          if record.exc_info:
              record.exc_text = self.formatException(record.exc_info, record)
          return logging.Formatter.format(self, record)
docs: Uses contextstring if request_id is set, otherwise default.