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

[Jim Fulton]

Lately I've been re-learning how to write new types in C.  Things
changed drastically (for the better) in 2.2. I've been updating the
documentation on writing new types as I go:

http://www.python.org/dev/doc/devel/ext/defining-new-types.html

(I'm also updating modulator.)

I'm starting to try to figure out how to integrate support for GC.
The current documentation in the section "Supporting the Cycle
Collector" doesn't reflect new-style types and is, thus, out of date.

Frankly, I'm taking the approach that there is only One Way to create
types in C, the new way, based on new-style types as now documented
in the manual.

I'll also note that most new-style types don't need and thus don't
implement custom allocators. They leave the tp_alloc and tp_free slots
empty.

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.)

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

Jim

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