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Objects/sliceobject.c

cpython 3.14 @ ab2d84fe1023/Objects/sliceobject.c

Objects/sliceobject.c defines PySliceObject and the Ellipsis singleton. Slices store start, stop, and step as Python objects (which may be None). The helper functions compute concrete integer indices from these objects given a sequence length.

Map​

LinesSymbolRole
1-80EllipsisObject, Py_EllipsisThe ... singleton
81-200PySlice_New, slice_reprSlice construction and display
201-320PySlice_GetIndicesExCompute concrete bounded indices (deprecated path)
321-440PySlice_Unpack, PySlice_AdjustIndicesNew 3.6+ split API

Reading​

PySlice_Unpack​

PySlice_Unpack(slice, &start, &stop, &step) converts None to default values without bounds-clamping. step defaults to 1; start defaults to 0 or PY_SSIZE_T_MAX depending on step sign; stop defaults to PY_SSIZE_T_MAX or PY_SSIZE_T_MIN.

// CPython: Objects/sliceobject.c:330 PySlice_Unpack
int
PySlice_Unpack(PyObject *_r,
               Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step)
{
    PySliceObject *r = (PySliceObject *)_r;
    if (r->step == Py_None) {
        *step = 1;
    } else {
        if (!_PyEval_SliceIndex(r->step, step)) return -1;
        if (*step == 0) {
            PyErr_SetString(PyExc_ValueError, "slice step cannot be zero");
            return -1;
        }
    }
    ...
}

PySlice_AdjustIndices​

PySlice_AdjustIndices(length, &start, &stop, step) clamps start and stop into [0, length] and returns the number of elements in the slice. This is the second step after PySlice_Unpack; together they replace the older single-call PySlice_GetIndicesEx.

Ellipsis​

Py_Ellipsis is the singleton .... It has no state; PyEllipsis_Type only defines __repr__ as "Ellipsis". It is used in extended slicing (a[..., 0]) and as a type annotation placeholder.

gopy notes​

PySliceObject is implemented in the core objects layer. PySlice_Unpack and PySlice_AdjustIndices are called by __getitem__ implementations for list, tuple, str, and bytes when the index is a slice object.