PEP 412 shared keys are not created for subclasses in Python 3.3 and 3.4:

>>> import sys
>>> class A:
...     pass
... 
>>> class B(A):
...     pass
... 
>>> a, b = A(), B()
>>> sys.getsizeof(vars(a))
96
>>> sys.getsizeof(vars(b))
288

(Actual sizes depend on platform and configuration).

This patch allows subclasses to share keys:

>>> import sys
>>> class A:
...     pass
... 
>>> class B(A):
...     pass
... 
>>> a, b = A(), B()
>>> sys.getsizeof(vars(a))
96
>>> sys.getsizeof(vars(b))
96

Wow, really? Thanks for finding this!

Well spotted.

I don't think that the line 
COPYVAL(tp_dictoffset);
should be removed.

inherit_special() is called from PyType_Ready
which is used to initialise static TypeObjects.
So builtin class B which doesn't set tp_dictoffset,
but has builtin class A as its base, will not get its
tp_dictoffset initialized from A.

Thanks Mark, I agree.

I thought that assigning to tp_dictoffset in inherit_special was redundant with the assignment in inherit_slot and the assignment I added, but I see that COPYSLOT and COPYVAL are slightly different and extension types won't go through type_new.

Here's an updated patch.

This looks good to me.

Also, I think this should go into 3.4
It is a tiny patch and could result in significant memory saving for OO programs with inheritance (e.g. Django)

> Also, I think this should go into 3.4

I think that it's too late for optimizations in Python 3.4 (RC1 is alread out). It can wait Python 3.5, or maybe 3.4.1.

I agree: definitely not for 3.5, maybe for 3.4.1.

New changeset 16229573e73e by Antoine Pitrou in branch 'default':
Issue #20637: Key-sharing now also works for instance dictionaries of subclasses.  Patch by Peter Ingebretson.
http://hg.python.org/cpython/rev/16229573e73e

Thanks. I've committed a patch after augmenting the tests a bit, so it'll be in 3.4.1 as well as 3.5.

New changeset afae24cb81d9 by Benjamin Peterson in branch '3.4':
correct the fix for #20637; allow slot descriptor inheritance to take place before creating cached keys
http://hg.python.org/cpython/rev/afae24cb81d9