Notes for myself.

the visit_decref() has been updated to display repr() of the parent object, rather than display the "freed" object.

Before:
---
Modules/gcmodule.c:378: visit_decref: Assertion "!_PyObject_IsFreed(op)" failed
<object at 0x5557629ca260 is freed>
Fatal Python error: _PyObject_AssertFailed
Python runtime state: preinitialized

Current thread 0x00007f6e2863e740 (most recent call first):
<no Python frame>
---

After:
---
Modules/gcmodule.c:378: visit_decref: Assertion "!_PyObject_IsFreed(op)" failed
Enable tracemalloc to get the memory block allocation traceback

object address  : 0x7fe2d1a52b40
object refcount : 1
object type     : 0x740be0
object type name: dict
object repr     : {'__repr__': <slot wrapper '__repr__' of 'weakref' objects>, '__hash__': <slot wrapper '__hash__' of 'weakref' objects>, '__call__': <slot wrapper '__call__' of 'weakref' objects>, '__lt__': <slot wrapper '__lt__' of 'weakref' objects>, '__le__': <slot wrapper '__le__' of 'weakref' objects>, '__eq__': <slot wrapper '__eq__' of 'weakref' objects>, '__ne__': <slot wrapper '__ne__' of 'weakref' objects>, '__gt__': <slot wrapper '__gt__' of 'weakref' objects>, '__ge__': <slot wrapper '__ge__' of 'weakref' objects>, '__init__': <slot wrapper '__init__' of 'weakref' objects>, '__new__': <built-in method __new__ of type object at 0x749500>, '__callback__': <member '__callback__' of 'weakref' objects>, '__doc__': None}

Fatal Python error: _PyObject_AssertFailed
Python runtime state: preinitialized

Current thread 0x00007fe2dea35740 (most recent call first):
<no Python frame>
---

The problem was that the None object was seen as "freed" because _ob_prev=_ob_next=NULL. This object doesn't seem to be part of the list of all objects, sys.getobjects(). I fixed Py_None, but then I got the same assertion error on a different (moduledef) object. I gave up, and Pablo and me fixed _PyObject_IsFreed() instead.