A few days ago, Guido said (in the thread "[Python-Dev] Python

version of PySlice_GetIndicesEx"):

> OK.  Michael, if you want to check in indices(), go ahead.

Then I did what was needed, but didn't check it in.  Here it is.

Doc/ref/ref3.tex

@@ -892,6 +892,15 @@ Special read-only attributes: \member{start} is the lower bound;
   \ttindex{stop}
   \ttindex{step}}
 
+Special method: \method{indices} takes an single integer argument
+\var{length} and computes information about the extended slice that
+the slice object would describe if applied to a sequence of
+\var{length}.  It returns a tuple of three integers; respectively
+these are the \var{start} and \var{stop} indices and the \var{step} or
+stride length of the slice.  Missing or out-of-bounds indices are
+handled in a manner consistent with regular slices.
+\versionadded{2.3}
+
 \end{description} % Internal types
 
 \end{description} % Types

Lib/test/test_slice.py

+# tests for slice objects; in particular the indices method.
+
+from test_support import vereq
+
+vereq(slice(None           ).indices(10), (0, 10,  1))
+vereq(slice(None,  None,  2).indices(10), (0, 10,  2))
+vereq(slice(1,     None,  2).indices(10), (1, 10,  2))
+vereq(slice(None,  None, -1).indices(10), (9, -1, -1))
+vereq(slice(None,  None, -2).indices(10), (9, -1, -2))
+vereq(slice(3,     None, -2).indices(10), (3, -1, -2))
+vereq(slice(-100,  100     ).indices(10), slice(None).indices(10))
+vereq(slice(100,  -100,  -1).indices(10), slice(None, None, -1).indices(10))
+vereq(slice(-100L, 100L, 2L).indices(10), (0, 10,  2))
+

Objects/sliceobject.c

@@ -221,6 +221,39 @@ static PyMemberDef slice_members[] = {
 	{0}
 };
 
+static PyObject*
+slice_indices(PySliceObject* self, PyObject* len)
+{
+	int ilen, start, stop, step, slicelength;
+
+	ilen = PyInt_AsLong(len);
+
+	if (ilen == -1 && PyErr_Occurred()) {
+		return NULL;
+	}
+
+	if (PySlice_GetIndicesEx(self, ilen, &start, &stop, 
+				 &step, &slicelength) < 0) {
+		return NULL;
+	}
+
+	return Py_BuildValue("(lll)", start, stop, step);
+}
+
+PyDoc_STRVAR(slice_indices_doc,
+"S.indices(len) -> (start, stop, stride)\n\
+\n\
+Assuming a sequence of length len, calculate the start and stop\n\
+indices, and the stride length of the extended slice described by\n\
+S. Out of bounds indices are clipped in a manner consistent with the\n\
+handling of normal slices.");
+
+static PyMethodDef slice_methods[] = {
+	{"indices",	(PyCFuntion)slice_indices,
+	 METH_O,	slice_indices_doc},
+	{NULL, NULL}
+};
+
 static int
 slice_compare(PySliceObject *v, PySliceObject *w)
 {
@@ -271,7 +304,7 @@ PyTypeObject PySlice_Type = {
 	0,					/* tp_weaklistoffset */
 	0,					/* tp_iter */
 	0,					/* tp_iternext */
-	0,					/* tp_methods */
+	slice_methods,				/* tp_methods */
 	slice_members,				/* tp_members */
 	0,					/* tp_getset */
 	0,					/* tp_base */