    def __getitem__(self, key):
        if key == '':
            idx = self._last_index
            self._last_index += 1
            try:
                return self._args[idx]
            except LookupError:
                pass
            key = str(idx)
        return self._kwargs[key]
The software uses a function that accepts a format string as an argument, but the format string originates from an external source.