What does cpython do to help detect object cycles(reference counting)?

Question

From what I've read about cpython it seems like it does reference counting + something extra to detect/free objects pointing to each other.(Correct me if I'm wrong). Could someone explain the something extra? Also does this guarantee* no cycle leaking? If not is there any research into an algorithm proven to add to reference counting to make it never leak*? Would this be just running a non reference counting tracing gc every so often?

*discounting bugs and problems with modules using foreign function interface

Answer 1

As explained in the documentation for gc.garbage, there is no guarantee that no leaks occur; specifically, cyclic objects with __del__ methods are not collected by default. For such objects, the cyclic links have to be manually broken to enable further GC.

From what I understand by browsing the CPython sourcecode, the interpreter keeps references to all objects under its control. The "extra" garbage collector runs a mark-and-sweep-like algorithm through the heap, remembers for each object if it is reachable from the "outside" and, if not, deletes it. (The GC is generational, but it may be run explicitly from the gc module with a generation argument.)

The only efficient algorithm that I could think of that satisfies your criteria would indeed be a "full" GC algorithm to augment reference counting and this is what seems to be implemented in Python. I'm not an expert in these matters though.