Function get_initial_from_ufunc

numpy/core/src/umath/legacy_array_method.c:265–327

* The default `get_reduction_initial` attempts to look up the identity * from the calling ufunc. This might fail, so we only call it when necessary. * * For internal number dtypes, we can easily cache it, so do so after the * first call by overriding the function with `copy_cache_initial`. * This path is not publicly available. That could be added, and for a * custom initial getter it shou

 * custom initial getter it should be static/compile time data anyway.
 */
static int
get_initial_from_ufunc(
        PyArrayMethod_Context *context, npy_bool reduction_is_empty,
        char *initial)
{
    if (context->caller == NULL
            || !PyObject_TypeCheck(context->caller, &PyUFunc_Type)) {
        /* Impossible in NumPy 1.24;  guard in case it becomes possible. */
        PyErr_SetString(PyExc_ValueError,
                "getting initial failed because it can only done for legacy "
                "ufunc loops when the ufunc is provided.");
        return -1;
    }
    npy_bool reorderable;
    PyObject *identity_obj = PyUFunc_GetDefaultIdentity(
            (PyUFuncObject *)context->caller, &reorderable);
    if (identity_obj == NULL) {
        return -1;
    }
    if (identity_obj == Py_None) {
        /* UFunc has no idenity (should not happen) */
        Py_DECREF(identity_obj);
        return 0;
    }
    if (PyTypeNum_ISUNSIGNED(context->descriptors[1]->type_num)
            && PyLong_CheckExact(identity_obj)) {
        /*
         * This is a bit of a hack until we have truly loop specific
         * identities.  Python -1 cannot be cast to unsigned so convert
         * it to a NumPy scalar, but we use -1 for bitwise functions to
         * signal all 1s.
         * (A builtin identity would not overflow here, although we may
         * unnecessary convert 0 and 1.)
         */
        Py_SETREF(identity_obj, PyObject_CallFunctionObjArgs(
                     (PyObject *)&PyLongArrType_Type, identity_obj, NULL));
        if (identity_obj == NULL) {
            return -1;
        }
    }
    else if (context->descriptors[0]->type_num == NPY_OBJECT
            && !reduction_is_empty) {
        /* Allows `sum([object()])` to work, but use 0 when empty. */
        Py_DECREF(identity_obj);
        return 0;
    }

    int res = PyArray_Pack(context->descriptors[0], initial, identity_obj);
    Py_DECREF(identity_obj);
    if (res < 0) {
        return -1;
    }

    if (PyTypeNum_ISNUMBER(context->descriptors[0]->type_num)) {
        /* For numbers we can cache to avoid going via Python ints */
        memcpy(context->method->legacy_initial, initial,
               context->descriptors[0]->elsize);
        context->method->get_reduction_initial = &copy_cached_initial;