[Python-Dev] Why a STACKADJ(-1) in UNARY_NOT on ceval.c? (original) (raw)

Cesare Di Mauro cesare.dimauro at a-tono.com
Thu Feb 5 19:19:01 CET 2009

Looking at the UNARY_NOT case in ceval.c:

    case UNARY_NOT:
        v = TOP();
        err = PyObject_IsTrue(v);
        Py_DECREF(v);
        if (err == 0) {
            Py_INCREF(Py_True);
            SET_TOP(Py_True);
            continue;
        }
        else if (err > 0) {
            Py_INCREF(Py_False);
            SET_TOP(Py_False);
            err = 0;
            continue;
        }
        STACKADJ(-1);
        break;

I don't understand why there's a STACKADJ(-1) at its end. Looking at the code, we know that if the CPU arrives to the STACKADJ, it's because of an error condition in the PyObject_IsTrue that sets err to a < 0 value, so exiting the big switch statement, an error will be raised.

So the question is, why there's the need to skip the top stack PyObject? It's a different behaviour comparing it to the all other unary operators. For example:

    case UNARY_NEGATIVE:
        v = TOP();
        x = PyNumber_Negative(v);
        Py_DECREF(v);
        SET_TOP(x);
        if (x != NULL) continue;
        break;

There's no STACKADJ instruction on errors.

Can someone explain it?

Thanks a lot

Cesare

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