importgc,weakrefclassLateFin:__slots__= ('ref',)def__del__(self):# 8. Now `latefin`'s finalizer is called. Here we obtain a reference to# `func`, which is currently undergoing `tp_clear`.globalfuncfunc=self.ref()classCyclic(tuple):__slots__= ()# 4. The finalizers of all garbage objects are called. In this case this is# only us as `func` doesn't have a finalizer.def__del__(self):# 5. Create a weakref to `func` now. If we had created it earlier, it# would have been cleared by the garbage collector before calling the# finalizers.self[1].ref=weakref.ref(self[0])# 6. Drop the global reference to `latefin`. The only remaining# reference is the one we have.globallatefindellatefin# 7. Now `func` is `tp_clear`-ed. This drops the last reference to `Cyclic`,# which gets `tp_dealloc`-ed. This drops the last reference to `latefin`.latefin=LateFin()func=lambda:Nonecyc=tuple.__new__(Cyclic, (func,latefin))# 1. Create a reference cycle of `cyc` and `func`.func.__module__=cyc# 2. Make the cycle unreachable, but keep the global reference to `latefin` so# that it isn't detected as garbage. This way its finalizer will not be called# immediately.delfunc,cyc# 3. Invoke garbage collection, which will find `cyc` and `func` as garbage.gc.set_debug(gc.DEBUG_COLLECTABLE)gc.collect()gc.set_debug(0)# 9. Call `repr()`, which will try to use the NULL `func_qualname`.repr(func)

#5  0x00007ffff7d0a596 in __GI___assert_fail (assertion=assertion@entry=0x697e93 "obj && _PyUnicode_CHECK(obj)", file=file@entry=0x696a92 "Objects/unicodeobject.c", line=line@entry=3000,     function=function@entry=0x69e110 <__PRETTY_FUNCTION__.299> "unicode_fromformat_arg") at assert.c:101#6  0x0000000000538d8e in unicode_fromformat_arg (writer=writer@entry=0x7fffffffd9f0, f=0x682bf1 "U at %p>", f@entry=0x682bf0 "%U at %p>", vargs=vargs@entry=0x7fffffffda28)    at Objects/unicodeobject.c:3000#7  0x0000000000539098 in PyUnicode_FromFormatV (format=<optimized out>, vargs=vargs@entry=0x7fffffffda68) at Objects/unicodeobject.c:3109#8  0x00000000005391ab in PyUnicode_FromFormat (format=format@entry=0x682be6 "<function %U at %p>") at Objects/unicodeobject.c:3161#9  0x00000000004c2c7b in func_repr (op=<optimized out>) at Objects/funcobject.c:710#10 0x00000000004ea437 in PyObject_Repr (v=0x91d650) at Objects/object.c:434#11 0x0000000000571932 in builtin_repr (module=<optimized out>, obj=<optimized out>) at Python/bltinmodule.c:2252#12 0x00000000004e6c82 in cfunction_vectorcall_O (func=0x9a10f0, args=<optimized out>, nargsf=<optimized out>, kwnames=<optimized out>) at Objects/methodobject.c:514#13 0x00000000004a5700 in _PyObject_VectorcallTstate (tstate=0x915430 <_PyRuntime+166000>, callable=callable@entry=0x9a10f0, args=args@entry=0x9a4f98, nargsf=9223372036854775809,     kwnames=kwnames@entry=0x0) at ./Include/internal/pycore_call.h:92#14 0x00000000004a57cb in PyObject_Vectorcall (callable=callable@entry=0x9a10f0, args=args@entry=0x9a4f98, nargsf=<optimized out>, kwnames=kwnames@entry=0x0) at Objects/call.c:299#15 0x0000000000586b4b in _PyEval_EvalFrameDefault (tstate=0x915430 <_PyRuntime+166000>, frame=0x9a4f40, throwflag=<optimized out>) at Python/ceval.c:4765#16 0x000000000058af91 in _PyEval_EvalFrame (tstate=tstate@entry=0x915430 <_PyRuntime+166000>, frame=frame@entry=0x9a4f40, throwflag=throwflag@entry=0)    at ./Include/internal/pycore_ceval.h:66#17 0x000000000058b09c in _PyEval_Vector (tstate=tstate@entry=0x915430 <_PyRuntime+166000>, func=func@entry=0xa41dd0, locals=locals@entry=0xa5ca80, args=args@entry=0x0,     argcount=argcount@entry=0, kwnames=kwnames@entry=0x0) at Python/ceval.c:6387#18 0x000000000058b190 in PyEval_EvalCode (co=co@entry=0xaa0e70, globals=globals@entry=0xa5ca80, locals=locals@entry=0xa5ca80) at Python/ceval.c:1157#19 0x00000000005cb444 in run_eval_code_obj (tstate=tstate@entry=0x915430 <_PyRuntime+166000>, co=co@entry=0xaa0e70, globals=globals@entry=0xa5ca80, locals=locals@entry=0xa5ca80)    at Python/pythonrun.c:1713#20 0x00000000005cb4fb in run_mod (mod=mod@entry=0xaafda0, filename=filename@entry=0xa8dad0, globals=globals@entry=0xa5ca80, locals=locals@entry=0xa5ca80, flags=flags@entry=0x7fffffffdf08,     arena=arena@entry=0xa8dc20) at Python/pythonrun.c:1734#21 0x00000000005cb5bf in pyrun_file (fp=fp@entry=0x9c8d20, filename=filename@entry=0xa8dad0, start=start@entry=257, globals=globals@entry=0xa5ca80, locals=locals@entry=0xa5ca80,     closeit=closeit@entry=1, flags=0x7fffffffdf08) at Python/pythonrun.c:1629#22 0x00000000005cded0 in _PyRun_SimpleFileObject (fp=fp@entry=0x9c8d20, filename=filename@entry=0xa8dad0, closeit=closeit@entry=1, flags=flags@entry=0x7fffffffdf08)    at Python/pythonrun.c:439#23 0x00000000005ce078 in _PyRun_AnyFileObject (fp=fp@entry=0x9c8d20, filename=filename@entry=0xa8dad0, closeit=closeit@entry=1, flags=flags@entry=0x7fffffffdf08) at Python/pythonrun.c:78#24 0x00000000005e9d4b in pymain_run_file_obj (program_name=program_name@entry=0xa02210, filename=filename@entry=0xa8dad0, skip_source_first_line=0) at Modules/main.c:353#25 0x00000000005e9e49 in pymain_run_file (config=config@entry=0x8fb490 <_PyRuntime+59600>) at Modules/main.c:372#26 0x00000000005ea4e8 in pymain_run_python (exitcode=exitcode@entry=0x7fffffffe05c) at Modules/main.c:592#27 0x00000000005ea735 in Py_RunMain () at Modules/main.c:671#28 0x00000000005ea78a in pymain_main (args=args@entry=0x7fffffffe0a0) at Modules/main.c:701#29 0x00000000005ea80f in Py_BytesMain (argc=<optimized out>, argv=<optimized out>) at Modules/main.c:725#30 0x000000000041d70f in main (argc=<optimized out>, argv=<optimized out>) at ./Programs/python.c:15