Unpickler_set_memo() crashes when the unpickler's memo attribute is set to a dict with negative numbers. The descriptor uses _Unpickler_MemoPut() which uses the dict key as index to a C array.

Python 3.3.0 (v3.3.0:bd8afb90ebf2, Feb  8 2013, 00:38:29) 
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys, pickle
>>> p = pickle.Unpickler(sys.stdin)
>>> p.memo = {-1: None}
segfault

The issue was found be Coverity Scan:

CID 486776 (#1 of 1): Improper use of negative value (NEGATIVE_RETURNS)
negative_returns: Passing variable "idx" to a parameter that cannot be negative.
5955            if (_Unpickler_MemoPut(self, idx, value) < 0)

New changeset 17b7af660f82 by Christian Heimes in branch '3.3':
Issue #18339: Negative ints keys in unpickler.memo dict no longer cause a
http://hg.python.org/cpython/rev/17b7af660f82

New changeset f3372692ca20 by Christian Heimes in branch 'default':
Issue #18339: Negative ints keys in unpickler.memo dict no longer cause a
http://hg.python.org/cpython/rev/f3372692ca20

Sorry that I was late with review. Here is some nitpicks from me.

Using assertRaises() as context manager in this case looks cleaner to me:

    with self.assertRaises(ValueError):
        unpickler.memo = {-1: None}

Moving the `if (idx == -1 && PyErr_Occurred())` check inside the `if (idx < 0)` block will increase the perfomance a little.

New changeset fa0a03afe359 by Christian Heimes in branch '3.3':
Issue #18339: use with self.assertRaises() to make test case more readable
http://hg.python.org/cpython/rev/fa0a03afe359

New changeset de44cd866bb0 by Christian Heimes in branch 'default':
Issue #18339: use with self.assertRaises() to make test case more readable
http://hg.python.org/cpython/rev/de44cd866bb0

I don't think that a reordered idx < 0 check is going to make any measurable difference. I like the separate checks as they make the code easier to understand. The first check tests for error in PyLong_AsSsize_t() and the second one checks for positive integers.