Returns the concrete content for a file descriptor.

        BACKWARD COMPATIBILITY: if you ask for file descriptors 0 1 or 2, it will return the data from stdin, stdout,
        or stderr as a flat string.

        :param fd:  A file descriptor.
        :return:    The concrete content.
        :rtype:     str

def dumps(self, fd, **kwargs):
        """
        Returns the concrete content for a file descriptor.

        BACKWARD COMPATIBILITY: if you ask for file descriptors 0 1 or 2, it will return the data from stdin, stdout,
        or stderr as a flat string.

        :param fd:  A file descriptor.
        :return:    The concrete content.
        :rtype:     str
        """
        if 0 <= fd <= 2:
            data = [self.stdin, self.stdout, self.stderr][fd].concretize(**kwargs)
            if type(data) is list:
                data = b''.join(data)
            return data
        return self.get_fd(fd).concretize(**kwargs)