Issue 33909: PyObject_CallFinalizerFromDealloc is not referenced in any documentation (original) (raw)

PEP 442 states that:

Two new C API functions are provided to ease calling of tp_finalize, especially from custom deallocators.

But it does not give the names of these functions, nor do any python docs I can discover.

From grepping for tp_finalize, it seems the functions in question are:

• PyObject_CallFinalizerFromDealloc
• PyObject_CallFinalizer

It would be great if these could be documented, and perhaps even an example given of when it's appropriate to call each one.

I think I can describe what two function does/for. But I'm not good English writer. Would you write document instead?

"Finalizer" is del in Python and tp_finalize in C.

Calling finalizer requires some tricks, including avoiding call twice. That is what PyObject_CallFinalizer() does. Use it instead of call type->tp_finalize(obj) directly.

But finalizer is called from destructor (tp_dealloc) usually, and calling finalizer from destructor need more hack (e.g. temporary increment refcount). This is what PyObject_CallFinalizerFromDealloc does.

Generally speaking, this API is used only when you want to use both of custom tp_dealloc and tp_finalize. This is very rare because you can write cleanup code in tp_dealloc and skip using tp_finalize.

When you need really need both of tp_dealloc and tp_finalize, note that:

• Finalizer may resurrect the object.
• When the type is subclassed from Python, tp_finalize is called automatically. You must not call PyObject_CallFinalizerFromDealloc then.

This is tp_dealloc of Future object in asyncio. This object uses both of tp_dealloc and tp_finalize for compatibility with Future object implemented in pure Python. This covers many edge cases of tp_dealloc.

static void
FutureObj_dealloc(PyObject *self)
{
    FutureObj *fut = (FutureObj *)self;

    if (Future_CheckExact(fut)) {
        /* When fut is subclass of Future, finalizer is called from
         * subtype_dealloc.
         */
        if (PyObject_CallFinalizerFromDealloc(self) < 0) {
            // resurrected.
            return;
        }
    }

    // Weakref callback and finalizer of other objects may start GC.
    // So you need to untrack this object from GC before calling PyObject_ClearWeakRefs()
    // or any Py_DECREF()s
    // Otherwise, GC will cause segfault because refcount of this object is 0.
    PyObject_GC_UnTrack(self);

    // Handle weakrefs
    if (fut->fut_weakreflist != NULL) {
        PyObject_ClearWeakRefs(self);
    }

    // Reuse tp_clear() for clearing all members.
    (void)FutureObj_clear(fut);

    // Free this object at last.
    Py_TYPE(fut)->tp_free(fut);
}