"O?" format was introduced by r4301 (15 years, 9 months ago). PyArg_ParseTuple() was first documented in Python 2.2, but without the full list of all formats. The format list was added to Python 2.3, but "O?" was never documented. So I get that no third party progam use it.

In Python trunk, "O?" is no more used and so I propose to remove it to simplify getarg.c (simplify getarg cannot be a bad thing). PyArg_ParseTuple() has now better formats than "O?".

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"O&" calls a callback: if the callback returns 0, raise an error; otherwise gets the object. Example of callbacks: PyXXX_Check() function (eg. PyInt_Check()).

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The full Python test suite pass without any error on a patched Python (trunk).