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/bufferobject.c

@@ -472,6 +472,61 @@ buffer_slice(PyBufferObject *self, Py_ssize_t left, Py_ssize_t right)
 					  right - left);
 }
 
+static PyObject *
+buffer_subscript(PyBufferObject *self, PyObject *item)
+{
+	void *p;
+	Py_ssize_t size;
+	
+	if (!get_buf(self, &p, &size, ANY_BUFFER))
+		return NULL;
+	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 += size;
+		return buffer_item(self, i);
+	}
+	else if (PySlice_Check(item)) {
+		Py_ssize_t start, stop, step, slicelength, cur, i;
+
+		if (PySlice_GetIndicesEx((PySliceObject*)item, size,
+				 &start, &stop, &step, &slicelength) < 0) {
+			return NULL;
+		}
+
+		if (slicelength <= 0)
+			return PyString_FromStringAndSize("", 0);
+		else if (step == 1)
+			return PyString_FromStringAndSize((char *)p + start,
+							  stop - start);
+		else {
+			PyObject *result;
+			char *source_buf = (char *)p;
+			char *result_buf = (char *)PyMem_Malloc(slicelength);
+
+			if (result_buf == NULL)
+				return PyErr_NoMemory();
+
+			for (cur = start, i = 0; i < slicelength;
+			     cur += step, i++) {
+				result_buf[i] = source_buf[cur];
+			}
+
+			result = PyString_FromStringAndSize(result_buf,
+							    slicelength);
+			PyMem_Free(result_buf);
+			return result;
+		}
+	}
+	else {
+		PyErr_SetString(PyExc_TypeError,
+				"sequence index must be integer");
+		return NULL;
+	}
+}
+
 static int
 buffer_ass_item(PyBufferObject *self, Py_ssize_t idx, PyObject *other)
 {
@@ -581,6 +636,98 @@ buffer_ass_slice(PyBufferObject *self, Py_ssize_t left, Py_ssize_t right, PyObje
 	return 0;
 }
 
+static int
+buffer_ass_subscript(PyBufferObject *self, PyObject *item, PyObject *value)
+{
+	PyBufferProcs *pb;
+	void *ptr1, *ptr2;
+	Py_ssize_t selfsize;
+	Py_ssize_t othersize;
+
+	if ( self->b_readonly ) {
+		PyErr_SetString(PyExc_TypeError,
+				"buffer is read-only");
+		return -1;
+	}
+
+	pb = value ? value->ob_type->tp_as_buffer : NULL;
+	if ( pb == NULL ||
+	     pb->bf_getreadbuffer == NULL ||
+	     pb->bf_getsegcount == NULL )
+	{
+		PyErr_BadArgument();
+		return -1;
+	}
+	if ( (*pb->bf_getsegcount)(value, NULL) != 1 )
+	{
+		/* ### use a different exception type/message? */
+		PyErr_SetString(PyExc_TypeError,
+				"single-segment buffer object expected");
+		return -1;
+	}
+	if (!get_buf(self, &ptr1, &selfsize, ANY_BUFFER))
+		return -1;
+	if (PyIndex_Check(item)) {
+		Py_ssize_t i = PyNumber_AsSsize_t(item, PyExc_IndexError);
+		if (i == -1 && PyErr_Occurred())
+			return -1;
+		if (i < 0)
+			i += selfsize;
+		return buffer_ass_item(self, i, value);
+	}
+	else if (PySlice_Check(item)) {
+		Py_ssize_t start, stop, step, slicelength;
+		
+		if (PySlice_GetIndicesEx((PySliceObject *)item, selfsize,
+				&start, &stop, &step, &slicelength) < 0)
+			return -1;
+
+		pb = value ? value->ob_type->tp_as_buffer : NULL;
+		if (pb == NULL ||
+		    pb->bf_getreadbuffer == NULL ||
+		    pb->bf_getsegcount == NULL) {
+			PyErr_BadArgument();
+			return -1;
+		}
+		if ((*pb->bf_getsegcount)(value, NULL) != 1) {
+			/* ### use a different exception type/message? */
+			PyErr_SetString(PyExc_TypeError,
+					"single-segment buffer object expected");
+			return -1;
+		}
+		if ((othersize = (*pb->bf_getreadbuffer)(value, 0, &ptr2)) < 0)
+			return -1;
+
+		if (othersize != slicelength) {
+			PyErr_SetString(
+				PyExc_TypeError,
+				"right operand length must match slice length");
+			return -1;
+		}
+
+		if (slicelength == 0)
+			return 0;
+		else if (step == 1) {
+			memcpy((char *)ptr1 + start, ptr2, slicelength);
+			return 0;
+		}
+		else {
+			Py_ssize_t cur, i;
+			
+			for (cur = start, i = 0; i < slicelength;
+			     cur += step, i++) {
+				((char *)ptr1)[cur] = ((char *)ptr2)[i];
+			}
+
+			return 0;
+		}
+	} else {
+		PyErr_SetString(PyExc_TypeError,
+				"buffer indices must be integers");
+		return -1;
+	}
+}
+
 /* Buffer methods */
 
 static Py_ssize_t
@@ -656,6 +803,12 @@ static PySequenceMethods buffer_as_sequence = {
 	(ssizessizeobjargproc)buffer_ass_slice, /*sq_ass_slice*/
 };
 
+static PyMappingMethods buffer_as_mapping = {
+	(lenfunc)buffer_length,
+	(binaryfunc)buffer_subscript,
+	(objobjargproc)buffer_ass_subscript,
+};
+
 static PyBufferProcs buffer_as_buffer = {
 	(readbufferproc)buffer_getreadbuf,
 	(writebufferproc)buffer_getwritebuf,
@@ -676,7 +829,7 @@ PyTypeObject PyBuffer_Type = {
 	(reprfunc)buffer_repr,			/* tp_repr */
 	0,					/* tp_as_number */
 	&buffer_as_sequence,			/* tp_as_sequence */
-	0,					/* tp_as_mapping */
+	&buffer_as_mapping,			/* tp_as_mapping */
 	(hashfunc)buffer_hash,			/* tp_hash */
 	0,					/* tp_call */
 	(reprfunc)buffer_str,			/* tp_str */