Function einsum_list_to_subscripts

numpy/core/src/multiarray/multiarraymodule.c:2815–2896

* Converts a list of subscripts to a string. * * Returns -1 on error, the number of characters placed in subscripts * otherwise. */

 * otherwise.
 */
static int
einsum_list_to_subscripts(PyObject *obj, char *subscripts, int subsize)
{
    int ellipsis = 0, subindex = 0;
    npy_intp i, size;
    PyObject *item;

    obj = PySequence_Fast(obj, "the subscripts for each operand must "
                               "be a list or a tuple");
    if (obj == NULL) {
        return -1;
    }
    size = PySequence_Size(obj);

    for (i = 0; i < size; ++i) {
        item = PySequence_Fast_GET_ITEM(obj, i);
        /* Ellipsis */
        if (item == Py_Ellipsis) {
            if (ellipsis) {
                PyErr_SetString(PyExc_ValueError,
                        "each subscripts list may have only one ellipsis");
                Py_DECREF(obj);
                return -1;
            }
            if (subindex + 3 >= subsize) {
                PyErr_SetString(PyExc_ValueError,
                        "subscripts list is too long");
                Py_DECREF(obj);
                return -1;
            }
            subscripts[subindex++] = '.';
            subscripts[subindex++] = '.';
            subscripts[subindex++] = '.';
            ellipsis = 1;
        }
        /* Subscript */
        else {
            npy_intp s = PyArray_PyIntAsIntp(item);
            /* Invalid */
            if (error_converting(s)) {
                PyErr_SetString(PyExc_TypeError,
                        "each subscript must be either an integer "
                        "or an ellipsis");
                Py_DECREF(obj);
                return -1;
            }
            npy_bool bad_input = 0;

            if (subindex + 1 >= subsize) {
                PyErr_SetString(PyExc_ValueError,
                        "subscripts list is too long");
                Py_DECREF(obj);
                return -1;
            }

            if (s < 0) {
                bad_input = 1;
            }