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

Jim Fulton jim@zope.com
Fri, 16 May 2003 16:22:53 -0400

Tim Peters wrote:

[Jim Fulton]

... I'll also note that most new-style types don't need and thus don't implement custom allocators. They leave the tpalloc and tpfree slots empty. I'm worried about half of that: tpfree is needed to release memory no matter whether obtained in a standard or custom way. I don't think tpfree slots always get filled in to something non-NULL by magic, and in the current Python source almost all new-style C types explicitly define a tpfree function (the exceptions are "strange" in some way). PEP 253 may be partly out of date here -- or not. In the section on creating a subclassable type, it says: """ The base type must do the following: - Add the flag value PyTPFLAGSBASETYPE to tpflags. - Declare and use tpnew(), tpalloc() and optional tpinit() slots. - Declare and use tpdealloc() and tpfree(). - Export its object structure declaration. - Export a subtyping-aware type-checking macro. """ This doesn't leave a choice about defining tpalloc() or tpfree() -- it says both are required. For a subclassable type, I believe both must actually be implemented too. For a non-subclassable type, I expect they're optional. But if you don't define tpfree in that case, then I believe you must also not do the obj->obtype->tpfree(obj) business in the tpdealloc slot (else it will segfault).

Hm, I didn't read the PEP, I just went by what Guido told me. :)

I was told that PyType_Ready fills in tp_alloc and tp_free with default values.

I updated the noddy example in the docs. In this example, I filled in neither tp_alloc or tp_free. I tested the examples and verified that they work.

I just added printf calls to verify that these slots are indeen null before the call to PyType_Ready and non-null afterwards.

Jim

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