[Python-Dev] Py_BuildValue and decref (original) (raw)

Mihai Ibanescu misa at redhat.com
Sat Sep 9 00:35:58 CEST 2006

On Fri, Sep 08, 2006 at 06:27:08PM -0400, Barry Warsaw wrote:

On Sep 8, 2006, at 6:06 PM, Mihai Ibanescu wrote: >There is no description of what happens when PyBuildValue fails. >Will it >decref the python object passed in? Will it not? I just want to point out that the C API documentation is pretty silent about the refcounting side-effects in error conditions (and often in success conditions too) of most Python functions. For example, what is the refcounting side-effects of PyDictSetItem() on val? What about if that function fails? Has val been incref'd or not? What about the side-effects on any value the new one replaces, both in success and failure?

In this particular case, it doesn't decref it (or so I read the code). Relevant code is in do_mkvalue from Python/modsupport.c

            case 'N':
            case 'S':
            case 'O':
            if (**p_format == '&') {
                    typedef PyObject *(*converter)(void *);
                    converter func = va_arg(*p_va, converter);
                    void *arg = va_arg(*p_va, void *);
                    ++*p_format;
                    return (*func)(arg);
            }
            else {
                    PyObject *v;
                    v = va_arg(*p_va, PyObject *);
                    if (v != NULL) {
                            if (*(*p_format - 1) != 'N')
                                    Py_INCREF(v);
                    }
                    else if (!PyErr_Occurred())
                            /* If a NULL was passed
                             * because a call that should
                             * have constructed a value
                             * failed, that's OK, and we
                             * pass the error on; but if
                             * no error occurred it's not
                             * clear that the caller knew
                             * what she was doing. */
                            PyErr_SetString(PyExc_SystemError,
                                    "NULL object passed to

Py_BuildValue"); return v; }

Barry, where can I ship you my cloning machine? :-)

Misa

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