On Mon, Dec 1, 2014 at 1:38 PM, Nathaniel Smith <n...@pobox.com> wrote:
> On Mon, Dec 1, 2014 at 4:06 AM, Guido van Rossum <gu...@python.org> wrote: > > On Sun, Nov 30, 2014 at 5:42 PM, Nathaniel Smith <n...@pobox.com> wrote: > >> > >> On Mon, Dec 1, 2014 at 1:27 AM, Guido van Rossum <gu...@python.org> > wrote: > >> > Nathaniel, did you look at Brett's LazyLoader? It overcomes the > subclass > >> > issue by using a module loader that makes all modules instances of a > >> > (trivial) Module subclass. I'm sure this approach can be backported as > >> > far > >> > as you need to go. > >> > >> The problem is that by the time your package's code starts running, > >> it's too late to install such a loader. Brett's strategy works well > >> for lazy-loading submodules (e.g., making it so 'import numpy' makes > >> 'numpy.testing' available, but without the speed hit of importing it > >> immediately), but it doesn't help if you want to actually hook > >> attribute access on your top-level package (e.g., making 'numpy.foo' > >> trigger a DeprecationWarning -- we have a lot of stupid exported > >> constants that we can never get rid of because our rules say that we > >> have to deprecate things before removing them). > >> > >> Or maybe you're suggesting that we define a trivial heap-allocated > >> subclass of PyModule_Type and use that everywhere, as a > >> quick-and-dirty way to enable __class__ assignment? (E.g., return it > >> from PyModule_New?) I considered this before but hesitated b/c it > >> could potentially break backwards compatibility -- e.g. if code A > >> creates a PyModule_Type object directly without going through > >> PyModule_New, and then code B checks whether the resulting object is a > >> module by doing isinstance(x, type(sys)), this will break. (type(sys) > >> is a pretty common way to get a handle to ModuleType -- in fact both > >> types.py and importlib use it.) So in my mind I sorta lumped it in > >> with my Option 2, "minor compatibility break". OTOH maybe anyone who > >> creates a module object without going through PyModule_New deserves > >> whatever they get. > > > > > > Couldn't you install a package loader using some install-time hook? > > > > Anyway, I still think that the issues with heap types can be overcome. > Hm, > > didn't you bring that up before here? Was the conclusion that it's > > impossible? > > I've brought it up several times but no-one's really discussed it :-). > That's because nobody dares to touch it. (Myself included -- I increased the size of typeobject.c from ~50 to ~5000 lines in a single intense editing session more than a decade ago, and since then it's been basically unmaintainable. :-( > I finally attempted a deep dive into typeobject.c today myself. I'm > not at all sure I understand the intricacies correctly here, but I > *think* __class__ assignment could be relatively easily extended to > handle non-heap types, and in fact the current restriction to heap > types is actually buggy (IIUC). > > object_set_class is responsible for checking whether it's okay to take > an object of class "oldto" and convert it to an object of class > "newto". Basically it's goal is just to avoid crashing the interpreter > (as would quickly happen if you e.g. allowed "[].__class__ = dict"). > Currently the rules (spread across object_set_class and > compatible_for_assignment) are: > > (1) both oldto and newto have to be heap types > (2) they have to have the same tp_dealloc > (3) they have to have the same tp_free > (4) if you walk up the ->tp_base chain for both types until you find > the most-ancestral type that has a compatible struct layout (as > checked by equiv_structs), then either > (4a) these ancestral types have to be the same, OR > (4b) these ancestral types have to have the same tp_base, AND they > have to have added the same slots on top of that tp_base (e.g. if you > have class A(object): pass and class B(object): pass then they'll both > have added a __dict__ slot at the same point in the instance struct, > so that's fine; this is checked in same_slots_added). > > The only place the code assumes that it is dealing with heap types is > in (4b) -- same_slots_added unconditionally casts the ancestral types > to (PyHeapTypeObject*). AFAICT that's why step (1) is there, to > protect this code. But I don't think the check actually works -- step > (1) checks that the types we're trying to assign are heap types, but > this is no guarantee that the *ancestral* types will be heap types. > [Also, the code for __bases__ assignment appears to also call into > this code with no heap type checks at all.] E.g., I think if you do > > class MyList(list): > __slots__ = () > > class MyDict(dict): > __slots__ = () > > MyList().__class__ = MyDict() > > then you'll end up in same_slots_added casting PyDict_Type and > PyList_Type to PyHeapTypeObjects and then following invalid pointers > into la-la land. (The __slots__ = () is to maintain layout > compatibility with the base types; if you find builtin types that > already have __dict__ and weaklist and HAVE_GC then this example > should still work even with perfectly empty subclasses.) > Have you filed this as a bug? I believe nobody has discovered this problem before. I've confirmed it as far back as 2.5 (I don't have anything older installed). > Okay, so suppose we move the heap type check (step 1) down into > same_slots_added (step 4b), since AFAICT this is actually more correct > anyway. This is almost enough to enable __class__ assignment on > modules, because the cases we care about will go through the (4a) > branch rather than (4b), so the heap type thing is irrelevant. > > The remaining problem is the requirement that both types have the same > tp_dealloc (step 2). ModuleType itself has tp_dealloc == > module_dealloc, while all(?) heap types have tp_dealloc == > subtype_dealloc. Yeah, I can't see a way that type_new() can create a type whose tp_dealloc isn't subtype_dealloc. > Here again, though, I'm not sure what purpose this > check serves. subtype_dealloc basically cleans up extra slots, and > then calls the base class tp_dealloc. So AFAICT it's totally fine if > oldto->tp_dealloc == module_dealloc, and newto->tp_dealloc == > subtype_dealloc, so long as newto is a subtype of oldto -- b/c this > means newto->tp_dealloc will end up calling oldto->tp_dealloc anyway. > I guess the simple check is an upper bound (or whatever that's called -- my math-speak is rusty ;-) for the necessary-and-sufficient check that you're describing. > OTOH it's not actually a guarantee of anything useful to see that > oldto->tp_dealloc == newto->tp_dealloc == subtype_dealloc, because > subtype_dealloc does totally different things depending on the > ancestry tree -- MyList and MyDict above pass the tp_dealloc check, > even though list.tp_dealloc and dict.tp_dealloc are definitely *not* > interchangeable. > > So I suspect that a more correct way to do this check would be something > like > > PyTypeObject *old__real_deallocer = oldto, *new_real_deallocer = newto; > while (old_real_deallocer->tp_dealloc == subtype_dealloc) > old_real_deallocer = old_real_deallocer->tp_base; > while (new_real_deallocer->tp_dealloc == subtype_dealloc) > new_real_deallocer = new_real_deallocer->tp_base; > if (old_real_deallocer->tp_dealloc != new_real_deallocer) > error out; > I'm not set up to disagree with you on this any more... > Module subclasses would pass this check. Alternatively it might make > more sense to add a check in equiv_structs that > (child_type->tp_dealloc == subtype_dealloc || child_type->tp_dealloc > == parent_type->tp_dealloc); I think that would accomplish the same > thing in a somewhat cleaner way. > > Obviously this code is really subtle though, so don't trust any of the > above without review from someone who knows typeobject.c better than > me! (Antoine?) > Or Benjamin? -- --Guido van Rossum (python.org/~guido)
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