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)

        try:
            # all other arguments are used as file names
            if self._bufsize:
                return open(string, self._mode, self._bufsize)
            else:
                return open(string, self._mode)
        except IOError:
            err = _sys.exc_info()[1]
            message = _("can't open '%s': %s")
            raise ArgumentTypeError(message % (string, err))