gdb traceback on "del self.unraisable":

(gdb) where
#0  _PyErr_WriteUnraisableDefaultHook (
    args=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <traceback at remote 0x7fffe9cfb5f0>, None, <_io.BufferedWriter at remote 0x7fffe9bcb890>)) at Python/errors.c:1294
#1  0x00000000005726ec in sys_unraisablehook (module=<module at remote 0x7fffeac10710>, 
    unraisable=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <traceback at remote 0x7fffe9cfb5f0>, None, <_io.BufferedWriter at remote 0x7fffe9bcb890>)) at ./Python/sysmodule.c:702
#2  0x0000000000435741 in _PyMethodDef_RawFastCallKeywords (method=0x7bb2e0 <sys_methods+1216>, self=<module at remote 0x7fffeac10710>, args=0x7ffffffd1d00, 
    nargs=1, kwnames=0x0) at Objects/call.c:650
#3  0x00000000004359f4 in _PyCFunction_Vectorcall (func=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00, 
    nargsf=1, kwnames=0x0) at Objects/call.c:737
#4  0x0000000000534666 in _PyObject_Vectorcall (callable=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00, 
    nargsf=1, kwnames=0x0) at ./Include/cpython/abstract.h:127
#5  0x00000000005346b1 in _PyObject_FastCall (func=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00, nargs=1)
    at ./Include/cpython/abstract.h:147
#6  0x0000000000536da4 in _PyErr_WriteUnraisableMsg (err_msg_str=0x0, obj=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at Python/errors.c:1379
#7  0x0000000000536ee6 in PyErr_WriteUnraisable (obj=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at Python/errors.c:1425


#8  0x00000000005eb97d in iobase_finalize (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at ./Modules/_io/iobase.c:299
#9  0x0000000000479fd2 in PyObject_CallFinalizer (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at Objects/object.c:307
#10 0x000000000047a044 in PyObject_CallFinalizerFromDealloc (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at Objects/object.c:325
#11 0x00000000005eb9e0 in _PyIOBase_finalize (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at ./Modules/_io/iobase.c:320


#12 0x00000000005f27d3 in buffered_dealloc (self=0x7fffe9bcb890) at ./Modules/_io/bufferedio.c:387
#13 0x000000000047ebff in _Py_Dealloc (op=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at Objects/object.c:2213
#14 0x00000000005f210a in _Py_DECREF (filename=0x71eef3 "./Modules/_io/bufferedio.c", lineno=2138, op=<_io.BufferedWriter at remote 0x7fffe9bcb890>)
    at ./Include/object.h:478
#15 0x00000000005f716a in bufferedrwpair_dealloc (self=0x7fffe9a90d70) at ./Modules/_io/bufferedio.c:2138


#16 0x000000000047ebff in _Py_Dealloc (op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>) at Objects/object.c:2213
#17 0x000000000048d1b6 in _Py_DECREF (filename=0x69fe40 "./Include/object.h", lineno=541, op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>)
    at ./Include/object.h:478
#18 0x000000000048d1e2 in _Py_XDECREF (op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>) at ./Include/object.h:541

# del UnraisableHookArgs

#19 0x000000000048d569 in structseq_dealloc (obj=0x7fffe9ad1360) at Objects/structseq.c:85
#20 0x000000000047ebff in _Py_Dealloc (
    op=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>))
    at Objects/object.c:2213
#21 0x0000000000463be6 in _Py_DECREF (filename=0x696dc9 "Objects/dictobject.c", lineno=1585, 
    op=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>))
    at ./Include/object.h:478
#22 0x0000000000468428 in delitem_common (mp=0x7fffe9b17950, hash=-822296730167156087, ix=0, 
    old_value=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>)) at Objects/dictobject.c:1585
#23 0x0000000000468663 in _PyDict_DelItem_KnownHash (op={'_old_hook': <built-in method unraisablehook of module object at remote 0x7fffeac10710>}, 
    key='unraisable', hash=-822296730167156087) at Objects/dictobject.c:1637
#24 0x00000000004684e5 in PyDict_DelItem (op={'_old_hook': <built-in method unraisablehook of module object at remote 0x7fffeac10710>}, key='unraisable')
    at Objects/dictobject.c:1603

# del self.unraisable

#25 0x000000000046ef21 in _PyObjectDict_SetItem (tp=0xa58830, dictptr=0x7fffe9a8ad30, key='unraisable', value=0x0) at Objects/dictobject.c:4614