When PY_SSIZE_T_CLEAN  is not #defined in Py3.10, PyArg_ParseTuple() etc. sets a SystemError but the return value says 1 (=SUCCESS)! 
=>  Causes terrific crashes with unfilled variables - instead of a clean Python exception.

Background: pywin32 suffers in masses from the drop of int support for the s#/y#/et#... formats in PyArg_ParseTuple() etc.  (PY_SSIZE_T_CLEAN is required in Py3.10). And only part of the source is already PY_SSIZE_T_CLEAN. Now it is very difficult to even run tests and weed out / check, because of freezes instead of nice Python exceptions.

=> Instead of preventing such freezes, the PY_SSIZE_T_CLEAN mechanism became the opposite: a clever trap, a sword of Damocles :)


The cause is in getargs.c:

=================== getargs.c / convertsimple() ====

#define REQUIRE_PY_SSIZE_T_CLEAN \
    if (!(flags & FLAG_SIZE_T)) { \
        PyErr_SetString(PyExc_SystemError, \
                        "PY_SSIZE_T_CLEAN macro must be defined for '#' formats"); \
        return NULL; \
    }
#define RETURN_ERR_OCCURRED return msgbuf

===================


=> The return NULL is further processed as no msg NULL -> no error.  
=> Perhaps it should be a `return converterr(...)` or `return sstc_system_error(...)` !?

Thank you for reporting.

@vstinner Can we provide a nice C traceback too?

I checked the warning, and now I think Python traceback is fine.

PyArg_Parse*() is used on top of the C function. So python traceback is enough to find which function is using '#' without Py_SSIZE_T_CLEAN.

New changeset c71d24f55828e7f0f2c8750d2e1b04d04539beff by Inada Naoki in branch 'master':
bpo-43321: Fix SystemError in getargs.c (GH-24656)
https://github.com/python/cpython/commit/c71d24f55828e7f0f2c8750d2e1b04d04539beff

Thanks for fixing my bug ;-) (bpo-40943)