[Python-Dev] GC, flags, and subtyping (original) (raw)

David Abrahams David Abrahams" <david.abrahams@rcn.com
Fri, 5 Apr 2002 00:02:17 -0500

The issue of http://aspn.activestate.com/ASPN/Mail/Message/python-dev/1041123 is back.

I had removed Py_TPFLAGS_HAVE_GC from my type, but as Thomas pointed out in http://aspn.activestate.com/ASPN/Mail/Message/python-dev/1039576, the flag is set again by PyType_Ready(). That would be fine if the tp_is_gc field, which is initially zero in my metatype object, were copied automatically from its base (PyTYpe_Type). As it stands, my metatype is "forced" to inherit a claim that it handles GC, but the machinery for determining whether a given instance of the metatype is GC-able is /not/ inherited. Thus, my statically-allocated base instance is treated as though it was allocated by the GC mechanism.

This seems unneccessarily dangerous to me, and I propose that we change the semantics of type inheritance to inherit tp_is_gc by default.

Further, I think this points out the need for clarification of the subtyping mechanism. There should be one place where I can see which flags and tp_xxx slots are inherited, what they mean, in addition to the answers to other mysteries hidden in typeobject.c et al. I'm willing to work something up, if I can get some guidance as to the most appropriate form. It's not clear to me that there's an appropriate place in the structure of the API docs (it's not even clear to me that the API docs are the right place for this).

-Dave

P.S. One proviso: I find TeX to be cumbersome at best and hair-pullingly frustrating at worst. If I had to write TeX I probably would just submit code comprehensibility patches and stop there. I'm probably going to start with code patches anyway...

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