argparse.py 文件源码

def __call__(self, string):
        # the special argument "-" means sys.std{in,out}
        if string == '-':
            if 'r' in self._mode:
                return _sys.stdin
            elif 'w' in self._mode:
                return _sys.stdout
            else:
                msg = _('argument "-" with mode %r' % self._mode)
                raise ValueError(msg)

        # all other arguments are used as file names
        if self._bufsize:
            return open(string, self._mode, self._bufsize)
        else:
            return open(string, self._mode)