-- replace then with slightly faster PyObject_Call(o,a,NULL).  (The
difference is that the latter requires a to be a tuple; the former
allows other values and wraps them in a tuple if necessary; it
involves two more levels of C function calls to accomplish all that.)

to inner scope, too.

copy the metatype from the base, the base actually has one!

1. You can now have __dict__ and/or __weakref__ in your __slots__
   (before only __weakref__ was supported).  This is treated
   differently than before: it merely sets a flag that the object
   should support the corresponding magic.

2. Dynamic types now always have descriptors __dict__ and __weakref__
   thrust upon them.  If the type in fact does not support one or the
   other, that descriptor's __get__ method will raise AttributeError.

3. (This is the reason for all this; it fixes SF bug 575229, reported
   by Cesar Douady.)  Given this code:
      class A(object): __slots__ = []
      class B(object): pass
      class C(A, B): __slots__ = []
   the class object for C was broken; its size was less than that of
   B, and some descriptors on B could cause a segfault.  C now
   correctly inherits __weakrefs__ and __dict__ from B, even though A
   is the "primary" base (C.__base__ is A).

4. Some code cleanup, and a few comments added.

types.  This prevents nonsense like 2.__class__ = bool or
True.__class__ = int.

trampolining going on with the tp_new descriptor, where the inherited
PyType_GenericNew was overwritten with the much slower slot_tp_new
which would end up calling tp_new_wrapper which would eventually call
PyType_GenericNew.  Add a special case for this to update_one_slot().

XXX Hope there isn't a loophole in this.  I'll buy the first person to
point out a bug in the reasoning a beer.

Backport candidate (but I won't do it).

intern the string "__new__" so we can call PyObject_GetAttr() rather
than PyObject_GetAttrString().  (Though it's a mystery why slot_tp_new
is being called when a class doesn't define __new__.  I'll look into
that tomorrow.)

2.2 backport candidate (but I won't do it).

a lot of work: it had to save and restore the current exception around
a call to lookup_maybe(), because that could fail in rare cases, and
most objects don't have a __del__ method, so the whole exercise was
usually a waste of time.  Changed this to cache the __del__ method in
the type object just like all other special methods, in a new slot
tp_del.  So now subtype_dealloc() can test whether tp_del is NULL and
skip the whole exercise if it is.  The new slot doesn't need a new
flag bit: subtype_dealloc() is only called if the type was dynamically
allocated by type_new(), so it's guaranteed to have all current slots.
Types defined in C cannot fill in tp_del with a function of their own,
so there's no corresponding "wrapper".  (That functionality is already
available through tp_dealloc.)

subtype_dealloc().

When call_finalizer() failed, it would return without going through
the trashcan end macro, thereby unbalancing the trashcan nesting level
counter, and thereby defeating the test case (slottrash() in
test_descr.py).  This in turn meant that the assert in the GC_UNTRACK
macro wasn't triggered by the slottrash() test despite a bug in the
code: _PyTrash_destroy_chain() calls the dealloc routine with an
object that's untracked, and the assert in the GC_UNTRACK macro would
fail on this; but because of an earlier test that resurrects an
object, causing call_finalizer() to fail and the trashcan nesting
level to be unbalanced, so _PyTrash_destroy_chain() was never called.
Calling the slottrash() test in isolation *did* trigger the assert,
however.

So the fix is twofold: (1) call the GC_UnTrack() function instead of
the GC_UNTRACK macro, because the function is safe when the object is
already untracked; (2) when call_finalizer() fails, jump to a label
that exits through the trashcan end macro, keeping the trashcan
nesting balanced.

This is inspired by SF patch 581742 (by Jonathan Hogg, who also
submitted the bug report, and two other suggested patches), but
separates the non-GC case from the GC case to avoid testing for GC
several times.

Had to fix an assert() from call_finalizer() that asserted that the
object wasn't untracked, because it's possible that the object isn't
GC'ed!

The __delete__ method wrapper for descriptors was not supported

(I added a test, too.)

2.2 bugfix candidate.

Fixes SF bug #575773.

Bug fix candidate.

The staticforward define was needed to support certain broken C
compilers (notably SCO ODT 3.0, perhaps early AIX as well) botched the
static keyword when it was used with a forward declaration of a static
initialized structure.  Standard C allows the forward declaration with
static, and we've decided to stop catering to broken C compilers.  (In
fact, we expect that the compilers are all fixed eight years later.)

I'm leaving staticforward and statichere defined in object.h as
static.  This is only for backwards compatibility with C extensions
that might still use it.

XXX I haven't updated the documentation.

If the object is an ExtensionClass, for example, the slot is not even
defined.  So we must check that the type has the slot (implied by
HAVE_CLASS) before calling tp_init().

the right thing even if char is unsigned.

helper macros to something saner, and used them appropriately in other
files too, to reduce #ifdef blocks.

classobject.c, instance_dealloc():  One of my worst Python Memories is
trying to fix this routine a few years ago when COUNT_ALLOCS was defined
but Py_TRACE_REFS wasn't.  The special-build code here is way too
complicated.  Now it's much simpler.  Difference:  in a Py_TRACE_REFS
build, the instance is no longer in the doubly-linked list of live
objects while its __del__ method is executing, and that may be visible
via sys.getobjects() called from a __del__ method.  Tough -- the object
is presumed dead while its __del__ is executing anyway, and not calling
_Py_NewReference() at the start allows enormous code simplification.

typeobject.c, call_finalizer():  The special-build instance_dealloc()
pain apparently spread to here too via cut-'n-paste, and this is much
simpler now too.  In addition, I didn't understand why this routine
was calling _PyObject_GC_TRACK() after a resurrection, since there's no
plausible way _PyObject_GC_UNTRACK() could have been called on the
object by this point.  I suspect it was left over from pasting the
instance_delloc() code.  Instead asserted that the object is still
tracked.  Caution:  I suspect we don't have a test that actually
exercises the subtype_dealloc() __del__-resurrected-me code.

In a fresh interpreter, type.mro(tuple) would segfault, because
PyType_Ready() isn't called for tuple yet.  To fix, call
PyType_Ready(type) if type->tp_dict is NULL.

gotten from a weak reference to NULL instead of to None.  This caused
the following assert() to fail (but only in 2.2 in the debug build --
I have to find a better test case).  Will backport.

473985.  Through a subtle rearrangement of some members in the etype
struct (!), mapping methods are now preferred over sequence methods,
which is necessary to support str.__getitem__("hello", slice(4)) etc.

assert into PyType_Ready(): now that we're not clearing tp_dict, we
can assert that it's non-NULL again.

discovered that subtype_traverse must traverse the type if it is a
heap type, because otherwise some cycles involving a type and its
instance would not be collected.  Simplest example:
    while 1:
        class C(object): pass
        C.ref = C()
This program grows without bounds before this fix.  (It grows ever
slower since it spends ever more time in the collector.)

Simply adding the right visit() call to subtype_traverse() revealed
other problems.  With MvL's help we re-learned that type_clear()
doesn't have to clear *all* references, only the ones that may not be
cleared by other means.  Careful analysis (see comments in the code)
revealed that only tp_mro needs to be cleared.  (The previous checkin
to this file adds a test for tp_mro==NULL to _PyType_Lookup() that's
essential to prevent crashes due to tp_mro being NULL when
subtype_dealloc() tries to look for a __del__ method.)  The same kind
of analysis also revealed that subtype_clear() doesn't need to clear
the instance dict.

With this fix, a useful property of the collector is once again
guaranteed: a single gc.collect() call will clear out all garbage.
(It didn't always before, which put us on the track of this bug.)

Will backport to 2.2.

about the test case, slot_nb_power gets called on behalf of its second
argument, but with a non-None modulus it wouldn't check this, and
believes it is called on behalf of its first argument.  Fix this
properly, and get rid of the code in _PyType_Lookup() that tries to
call _PyType_Ready().  But do leave a check for a NULL tp_mro there,
because this can still legitimately occur.

I'll fix this in 2.2.x too.

While I was at it, I added a tp_clear handler and changed the
tp_dealloc handler to use the clear_slots helper for the tp_clear
handler.

Also tightened the rules for slot names: they must now be proper
identifiers (ignoring the dirty little fact that <ctype.h> is locale
sensitive).

Also set mp->flags = READONLY for the __weakref__ pseudo-slot.

Most of this is a 2.2 bugfix candidate; I'll apply it there myself.

_PyType_Lookup().  Decided to clear the error condition in the
unfortunate but unlikely case that PyType_Ready() fails.

Will fix in 2.2.x too.

whose tp_mro hasn't been initialized, it would dump core.  Fix this by
checking for NULL and calling PyType_Ready().  Will fix this in 2.2.1
too.

different free or alloc slot.

Change type_get_doc (the get function for __doc__) to look in tp_dict
more often, and if it finds a descriptor in tp_dict, to call it (with
a NULL instance).  This means you can add a __doc__ descriptor to a
new-style class that returns instance docs when called on an instance,
and class docs when called on a class -- or the same docs in either
case, but lazily computed.

I'll also check this into the 2.2 maintenance branch.

A stupid bug in object_set_class(): didn't check for value==NULL
before checking its type.

Bugfix candidate.

PyObject_Del and PyObject_GC_Del can now be used as a function
designators.

  when PyType_Ready() was called, if ob_type was found to be NULL, it
  was always set to &PyType_Type; now it is set to base->ob_type,
  where base is tp_base, defaulting to &PyObject_Type.

- PyType_Ready() accidentally did not inherit tp_is_gc; now it does.

Bugfix candidate.