The proposed patches move to test.support some functions, which will be useful for testing __sizeof__. The next few patches will use it. Also in 3.3 some bugs have fixed, now "n" is used for Py_ssize_t and Py_hash_t (which is not long now).

The problem is that this adds new features to test_support, which can only be done in 3.4.

Can we get around this by renaming check_sizeof into _check_sizeof? Or would 
we have to duplicate these auxiliary functions in every test file with 
__sizeof__ test?

Not sure: that's for release managers to decide.

I asked in Python-Dev, and if I understood the answers correctly, we can make changes to 2.7 and 3.2 without fear. For 3.3 risk is higher, so we need to be more attentive. However patch for 3.3 contains not only the potential improvements for other tests, but also bug fixes (I think these bugs should cause tests failure on the platforms where sizeof(Py_ssize_t) != sizeof(long) or sizeof(Py_ssize_t) != sizeof(void*)).

Of cause, if Georg Brandl not vetoes this.

Patches updated. Simplified struct __sizeof__ testing for 2.7 and 3.2 and fixed (using 'n' for Py_ssize_t) for 3.3.

New changeset efade142ef01 by Martin v. Löwis in branch '3.2':
Issue #15467: Move helpers for __sizeof__ tests into test_support.
http://hg.python.org/cpython/rev/efade142ef01

New changeset d43ff8eb4cb3 by Martin v. Löwis in branch 'default':
Issue #15467: Merge 3.2
http://hg.python.org/cpython/rev/d43ff8eb4cb3

New changeset fa95b04e67fd by Martin v. Löwis in branch '2.7':
Issue #15467: Move helpers for __sizeof__ tests into test_support.
http://hg.python.org/cpython/rev/fa95b04e67fd

I wasn't aware that test_support isn't public API. The patches are fine then; committed.