def get_stderr(self, os_path=None, skip_sub_command=False):
        """
        Returns a stderr-suitable file-like object based on the
        optional os_path and optionally skipping any configured
        sub-command.
        """
        sub_command = None if skip_sub_command else self.stderr_sub_command
        out, path = self._get_out_and_path(
            self.stderr, self.stderr_root, sub_command, os_path)
        if hasattr(out, 'stdin'):
            return out.stdin
        return out
Returns a stderr-suitable file-like object based on the
        optional os_path and optionally skipping any configured
        sub-command.