[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Stefan Behnel]

Stefan Behnel added the comment:

Just as a quick update here: Cython has since then switched to only using 
PyObject_ClearWeakRefs() and otherwise leaves the handling of the weakref slot 
to CPython.

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[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Phil Connell]

Changes by Phil Connell :


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nosy: +pconnell

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[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Jesús Cea Avión]

Changes by Jesús Cea Avión :


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nosy: +jcea

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[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Antoine Pitrou]

Antoine Pitrou added the comment:

I think Cython is wrong here. The only thing to do is to call 
PyObject_ClearWeakRefs() in the deallocator. Everything else is handled by the 
interpreter.

Agreed improving the documentation would be good.

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nosy: +pitrou, scoder
stage:  -> needs patch
type:  -> behavior
versions: +Python 2.7, Python 3.2, Python 3.3, Python 3.4

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[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Andrew Svetlov]

Changes by Andrew Svetlov :


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nosy: +asvetlov

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[issue16195] Difficult or impossible to figure out how garbage collector and weak references should interact for user-defined extension types

[Jean-Paul Calderone]

New submission from Jean-Paul Calderone:

There appears to be very little, if any, documentation about how to handle the 
list at tp_weaklistoffset for types supporting being weak referenced, 
particularly with respect to garbage collection.

Who owns the list?  Who owns the objects in the list?  Should the list be 
traversed by the extension type's tp_traverse?  Should it ever be INCREF'd or 
DECREF'd?  Does tp_dealloc have any responsibilities with respect to it?

>From looking at CPython's own source, it appears that tp_traverse should not 
>touch it (Yhg1s confirmed this on #python).  Cython, at least, appears to have 
>come to an alternate conclusion though - it generates tp_traverse functions 
>which visit the list (and provoke gc assertions to fail on Python 2.7 in debug 
>mode).

Some documentation about what is correct to do would be excellent, and would 
make it easier to explain to Cython developers what they're doing wrong (or 
confirm that they're doing something right, in which case there are some more 
bugs to file against the CPython stdlib, since they do something different from 
Cython).

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assignee: docs@python
components: Documentation
messages: 172639
nosy: docs@python, exarkun
priority: normal
severity: normal
status: open
title: Difficult or impossible to figure out how garbage collector and weak 
references should interact for user-defined extension types

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