Given two separately refcounted objects, how can you arrange for each of them to stay live so long as the other is live? Why doesn't just having a strong-strong or strong-weak reference between the two objects work? What is object resurrection in CPython? What's a finalizer and why does it make things more complicated? How does Python GC work?

Further reading.

• PyObject preservation PR https://github.com/pytorch/pytorch/pull/56017
• Sam Gross's original PoC, which works fine if the two objects in question are both PyObjects https://github.com/colesbury/refcount/
• PEP 442 Safe object finalization https://www.python.org/dev/peps/pep-0442/
• Essential reading about Python GC https://devguide.python.org/garbage_collector/