I’m trying to understand how Python’s garbage collector detects circular references. When I look at the documentation, all I see is a statement that circular references are detected, except when the objects involved have a __del__ method.
If this happens, my understanding (possibly faulty) is that the gc module acts as a failsafe by (I assume) walking through all the allocated memory and freeing any unreachable blocks.
How does Python detect & free circular memory references before making use of the gc module?
I think I found the answer I’m looking for in some links provided by @SvenMarnich in comments to the original question:
Container objects are Python objects that can hold references to other Python objects. Lists, Classes, Tuples etc are container objects; Integers, Strings etc. are not. So, only container objects are at risk for being in a circular reference.
Each Python object has a field – *gc_ref*, which is (I believe) set to NULL for non-container objects. For container objects it is set equal to the number of non container objects that reference it
Any container object with a *gc_ref* count greater than 1 (? I would’ve thought 0, but OK for now ?) has references that are not container objects. So they are reachable and are removed from consideration of being unreachable memory islands.
Any container object reachable by an object known to be reachable (i.e. those we just recognized as having a *gc_ref* count greater than 1) also does not need to be freed.
The remaining container objects are not reachable (except by each other) and should be freed.
http://www.arctrix.com/nas/python/gc/ is a link providing a fuller explanation
http://hg.python.org/cpython/file/2059910e7d76/Modules/gcmodule.c is a link to the source code, which has comments further explaining the thoughts behind the circular reference detection