python3 ctypes 的 结构体定义中，怎么引用自身？

我这么使用时提示说“找不到Sentence_link”

 class Sentence_link(ctypes.Structure):
        _fields_= [("para_idx", ctypes.c_ulong), ("sent_idx", ctypes.c_ulong),("sent_init",ctypes.c_int),("msg_buff",ctypes.c_char_p), ("next",ctypes.POINTER(Sentence_link))]

 _fields_= [("para_idx", ctypes.c_ulong), ("sent_idx", ctypes.c_ulong),("sent_init",ctypes.c_int),("msg_buff",ctypes.c_char_p), ("next",ctypes.POINTER(Sentence_link))]
NameError: name 'Sentence_link' is not defined

请问该如何引用自身？

https://bbs.csdn.net/topics/392444572