[Python-Dev] C new-style classes and GC (original) (raw)

Jeremy Hylton jeremy@zope.com
16 May 2003 12:42:03 -0400

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 PyTPFLAGSHAVEGC type flag, - Provide implementations of tptraverse and tpclear, as described in the section "Supporting the Cycle Collector" section of the docs. - Call PyObjectGCUnTrack 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 PyObjectGCNew, and PyObjectGCDEL, because the default tpalloc and tpfree take care of that for us. Similarly, we don't need to call PyObjectGCTrack, 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.

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.

This question isn't specific to GC. Perhaps it's a question of what tp_free is used for and when it should be called. Pure-Python classes and instances have tp_dealloc implementations that call tp_free. I'm not sure if that's a generic recommendation for all types written in C.

Did I get this right? I intend to update the docs to reflect this understanding (or a corrected one, of course).

The three items you listed were sufficient for all the types I've worked on, expecting the issues I noted above.

Jeremy