memory leaks - Does Python GC deal with reference-cycles like this?

Question

Using objgraph, I found a bunch of objects like this:

Will Python's garbage collector deal with cycles like this, or will it leak?

A slightly wider view of the loop:

Answer 1

Python's standard reference counting mechanism cannot free cycles, so the structure in your example would leak.

The supplemental garbage collection facility, however, is enabled by default and should be able to free that structure, if none of its components are reachable from the outside anymore and they do not have __del__() methods.

If they do, the garbage collector will not free them because it cannot determine a safe order to run these __del__() methods.