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

Tim Peters tim@zope.com
Fri, 16 May 2003 15:29:54 -0400

[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: tp_free is needed to release memory no matter whether obtained in a standard or custom way. I don't think tp_free 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 tp_free 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PEP 253 may be partly out of date here -- or not.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 Py_TPFLAGS_BASETYPE to tp_flags.

  - Declare and use tp_new(), tp_alloc() and optional tp_init()
    slots.

  - Declare and use tp_dealloc() and tp_free().

  - Export its object structure declaration.

  - Export a subtyping-aware type-checking macro.

"""

This doesn't leave a choice about defining tp_alloc() or tp_free() -- 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 tp_free in that case, then I believe you must also not do the

obj->ob_type->tp_free(obj)

business in the tp_dealloc slot (else it will segfault).