Let me summarize the issue: the PyArg_ParseTuple format code 'O' returns a borrowed reference.  However, when the 'O' code appears inside parenthesis, there may not be an object to hold the reference to borrow from.  This is what happens in the test-functools.py crasher:  partial.__setstate__() takes a 4-tuple argument that is unpacked using a "(OOOO)" format.  The test case passes an instance instead of a tuple that supports the sequence methods, but does not hold the reference to the "items" that its []-operator returns.  This is not a problem at the top level because args argument to PyArg_ParseTuple is always a real tuple.

I think that rather than deprecating the use of 'O' format inside parentheses, "(..O..)" unpacking should reject to unpack arguments other than tuples or maybe lists.