[Python-Dev] results of id() and weakref.getweakrefs() sometimes break on object resurrection

Stefan Richthofer Stefan.Richthofer at gmx.de
Sun Oct 26 00:20:47 CEST 2014

Hello developers,

I observed strange behaviour in CPython (tested in 2.7.5 and 3.3.3)
regarding object resurrection.
Yes, resurrection is evil, but it is a valid scenario. If an object is
resurrected via its finalizer __del__, sometimes its unique id value as
returned from id() changes. Additionally the list of weak references
pointing to it as returned by weakref.getweakrefs(...) breaks (i.e. is
suddenly empty, assuming it wasn't before).
These issues only arise if the resurrection occurs during
cyclic garbage collection. If the object is finalized because its refcount
drops to zero, everything is fine. See the attached test-file.

Is this behaviour intended or is it a bug? If so, which variant is the bug
and which is right? I can hardly believe that whether id() is preserved
should depend on whether the garbage was cyclic or not.

This might appear of low relevance to you, since no sane program intentionally
performs resurrection. However I originally became aware of the issue
in Jython (where it not only occurs for cyclic garbage but in every
resurrection-case), c.f. http://bugs.jython.org/issue2224.
I am interested in this because I am implementing native gc support
in JyNI and need to understand these details to do it right.

Thanks in advance!

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