[Python-Dev] C new-style classes and GC

[Jim Fulton]

Jeremy Hylton wrote:
> On Fri, 2003-05-16 at 11:46, Jim Fulton wrote:
> 
>>So given that we have a new style type, to add support for GC, we need
>>to:
>>
>>- Set the Py_TPFLAGS_HAVE_GC type flag,
>>
>>- Provide implementations of tp_traverse and tp_clear, as described in
>>   the section "Supporting the Cycle Collector" section of the docs.
>>
>>- Call PyObject_GC_UnTrack at the beginning of the deallocator,
>>   before decrefing any members.
>>
>>I think that that is *all* we have to do.
>>
>>In particular, since we have a new style type that inherits the
>>standard allocator, we don't need to fool with PyObject_GC_New, and
>>PyObject_GC_DEL, because the default tp_alloc and tp_free take care of
>>that for us. Similarly, we don't need to call PyObject_GC_Track,
>>because that is done by the default allocator. (Because of that, our
>>traverse function has to check for null object pointers in our
>>object's members.)
> 
> 
> It depends on how the objects are used in C code.  I've upgraded a lot
> of C extensions to make their types collectable recently.  In several
> cases, it was necessary to change PyObject_New to PyObject_GC_New and
> add a PyObject_GC_Track.  I think the docs ought to explain how to do
> this.

If you write types the New Way, there are no PyObject_New calls and
no need to call PyObject_GC_Track.

> It's not clear to me what the one right way to implement a tp_dealloc
> slot is.  I've seen two common patterns in the Python source: call
> obj->ob_type->tp_free or call PyObject_GC_Del.  The type object
> initializes tp_free to PyObject_GC_Del, so in most cases the two
> spellings are equivalent.  Calling PyObject_GC_Del feels more
> straightforward to me.

You need to call obj->ob_type->tp_free to support subclassing.

I suggest that every new type should call obj->ob_type->tp_free
as a matter of course.

Jim

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