If a class was defined inside a function, used a static or class
method, and used super() inside the method body, it would be caught in
an uncollectable cycle.  (Simplified version: The static/class method
object would point to a function object with a closure that referred
to the class.)

Bugfix candidate.

when DST began.

These never failed in 2.3, and the tests confirm it.  They still blow up
in the 2.2 branch, despite that all the gc-vs-__del__ fixes from 2.3
have been backported (and this is expected -- 2.2 needs more work than
2.3 needed).

Based on a suggestion from a reader.

cases, wrote docs, added a test.

of PyObject_HasAttr(); the former promises never to execute
arbitrary Python code.  Undid many of the changes recently made to
worm around the worst consequences of that PyObject_HasAttr() could
execute arbitrary Python code.

Compatibility is hard to discuss, because the dangerous cases are
so perverse, and much of this appears to rely on implementation
accidents.

To start with, using hasattr() to check for __del__ wasn't only
dangerous, in some cases it was wrong:  if an instance of an old-
style class didn't have "__del__" in its instance dict or in any
base class dict, but a getattr hook said __del__ existed, then
hasattr() said "yes, this object has a __del__".  But
instance_dealloc() ignores the possibility of getattr hooks when
looking for a __del__, so while object.__del__ succeeds, no
__del__ method is called when the object is deleted.  gc was
therefore incorrect in believing that the object had a finalizer.

The new method doesn't suffer that problem (like instance_dealloc(),
_PyObject_Lookup() doesn't believe __del__ exists in that case), but
does suffer a somewhat opposite-- and even more obscure --oddity:
if an instance of an old-style class doesn't have "__del__" in its
instance dict, and a base class does have "__del__" in its dict,
and the first base class with a "__del__" associates it with a
descriptor (an object with a __get__ method), *and* if that
descriptor raises an exception when __get__ is called, then
(a) the current method believes the instance does have a __del__,
but (b) hasattr() does not believe the instance has a __del__.

While these disagree, I believe the new method is "more correct":
because the descriptor *will* be called when the object is
destructed, it can execute arbitrary Python code at the time the
object is destructed, and that's really what gc means by "has a
finalizer":  not specifically a __del__ method, but more generally
the possibility of executing arbitrary Python code at object
destruction time.  Code in a descriptor's __get__() executed at
destruction time can be just as problematic as code in a
__del__() executed then.

So I believe the new method is better on all counts.

Bugfix candidate, but it's unclear to me how all this differs in
the 2.2 branch (e.g., new-style and old-style classes already
took different gc paths in 2.3 before this last round of patches,
but don't in the 2.2 branch).

out whether __del__ exists, without executing any Python-level code.

instead of looping.  Smaller and clearer.  Faster, too, when we're not
appending to gc.garbage:  gc_list_merge() takes constant time, regardless
of the lists' sizes.

append_objects():  Moved up to live with the other list manipulation
utilities.

mwh pointed out that the error message did not
make sense if obtained by rearranging the bases.

take no arguments; cuts generated code size.

externally unreachable objects with finalizers, and externally unreachable
objects without finalizers reachable from such objects.  This allows us
to call has_finalizer() at most once per object, and so limit the pain of
nasty getattr hooks.  This fixes the failing "boom 2" example Jeremy
posted (a non-printing variant of which is now part of test_gc), via never
triggering the nasty part of its __getattr__ method.

to special-case classic classes, or to worry about refcounts;
has_finalizer() deleted the current object iff the first entry in
the unreachable list has changed.  I don't believe it was correct
to check for ob_refcnt == 1, either:  the dealloc routine would get
called by Py_DECREF then, but there's nothing to stop the dealloc
routine from ressurecting the object, and then gc would remain at
the head of the unreachable list despite that its refcount temporarily
fell to 0 (and that would lead to an infinite loop in move_finalizers()).

I'm still worried about has_finalizer() resurrecting other objects
in the unreachable list:  what's to stop them from getting collected?

change (it would be another kind of bug if the trash cycle weren't
reclaimed).

squashed-together conditional operators; makes it much easier to step
thru in the debugger, and to set a breakpoint on the only dangerous
path.

delstr from initgc() into collect().  initgc() isn't called unless the
user explicitly imports gc, so can be used only for initialization of
user-visible module features; delstr needs to be initialized for proper
internal operation, whether or not gc is explicitly imported.

Bugfix candidate?  I don't know whether the new bug was backported to
2.2 already.

move_finalizers() moves every object from unreachable to collectable
or finalizers, unless the object is deallocated first.

opening it in non-blocking mode.  Both Guido and David Hammerton have
reported that this fixes their problems with ossaudiodev -- hooray!

an OverflowError instead of a TypeError to be consistent
with "%c" % 256. See SF patch #710127.