Improve extended slicing support in builtin types and classes. Specifically:

- Specialcase extended slices that amount to a shallow copy the same way as
   is done for simple slices, in the tuple, string and unicode case.

 - Specialcase step-1 extended slices to optimize the common case for all
   involved types.

 - For lists, allow extended slice assignment of differing lengths as long
   as the step is 1. (Previously, 'l[:2:1] = []' failed even though
   'l[:2] = []' and 'l[:2:None] = []' do not.)

 - Implement extended slicing for buffer, array, structseq, mmap and
   UserString.UserString.

 - Implement slice-object support (but not non-step-1 slice assignment) for
   UserString.MutableString.

 - Add tests for all new functionality.

Objects/structseq.c

@@ -89,6 +89,54 @@ structseq_slice(PyStructSequence *obj, Py_ssize_t low, Py_ssize_t high)
 	return (PyObject *) np;
 }
 
+static PyObject *
+structseq_subscript(PyStructSequence *self, PyObject *item)
+{
+	if (PyIndex_Check(item)) {
+		Py_ssize_t i = PyNumber_AsSsize_t(item, PyExc_IndexError);
+		if (i == -1 && PyErr_Occurred())
+			return NULL;
+
+		if (i < 0)
+			i += VISIBLE_SIZE(self);
+
+		if (i < 0 || i >= VISIBLE_SIZE(self)) {
+			PyErr_SetString(PyExc_IndexError,
+				"tuple index out of range");
+			return NULL;
+		}
+		Py_INCREF(self->ob_item[i]);
+		return self->ob_item[i];
+	}
+	else if (PySlice_Check(item)) {
+		Py_ssize_t start, stop, step, slicelen, cur, i;
+		PyObject *result;
+		
+		if (PySlice_GetIndicesEx((PySliceObject *)item,
+					 VISIBLE_SIZE(self), &start, &stop,
+					 &step, &slicelen) < 0) {
+			return NULL;
+		}
+		if (slicelen <= 0)
+			return PyTuple_New(0);
+		result = PyTuple_New(slicelen);
+		if (result == NULL)
+			return NULL;
+		for (cur = start, i = 0; i < slicelen;
+		     cur += step, i++) {
+			PyObject *v = self->ob_item[cur];
+			Py_INCREF(v);
+			PyTuple_SET_ITEM(result, i, v);
+		}
+		return result;
+	}
+	else {
+		PyErr_SetString(PyExc_TypeError,
+				"structseq index must be integer");
+		return NULL;
+	}
+}
+
 static PyObject *
 structseq_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
@@ -298,6 +346,11 @@ static PySequenceMethods structseq_as_sequence = {
 	(objobjproc)structseq_contains,	        /* sq_contains */
 };
 
+static PyMappingMethods structseq_as_mapping = {
+	(lenfunc)structseq_length,
+	(binaryfunc)structseq_subscript,
+};
+
 static PyMethodDef structseq_methods[] = {
 	{"__reduce__", (PyCFunction)structseq_reduce, 
 	 METH_NOARGS, NULL},
@@ -317,7 +370,7 @@ static PyTypeObject _struct_sequence_template = {
 	(reprfunc)structseq_repr,             	/* tp_repr */
 	0,					/* tp_as_number */
 	&structseq_as_sequence,			/* tp_as_sequence */
-	0,					/* tp_as_mapping */
+	&structseq_as_mapping,			/* tp_as_mapping */
 	structseq_hash,				/* tp_hash */
 	0,              			/* tp_call */
 	0,					/* tp_str */