code: def log(self, entry, *args):
          
          if args:
              entry = entry % args
          self.backup_log.append(entry)
docs: Append the string supplied to the log (a list of strings). If
          additional arguments are supplied, then first string is assumed to be
          a format string and the other args are used for string interpolation.
          For instance `backup.log("%d + %d == %d", 1, 1, 2)` would result in the
          string `'1 + 1 == 2'` being logged