This is my nearly two year old patch

[ 400998 ] experimental support for extended slicing on lists

somewhat spruced up and better tested than it was when I wrote it.

Includes docs & tests.  The whatsnew section needs expanding, and arrays
should support extended slices -- later.

Doc/api/concrete.tex

@@ -2288,6 +2288,32 @@ They are found in the dictionary of type objects.
 
 \begin{cfuncdesc}{int}{PySlice_GetIndices}{PySliceObject *slice, int length,
                                            int *start, int *stop, int *step}
+Retrieve the start, stop and step indices from the slice object
+\var{slice}, assuming a sequence of length \var{length}. Treats
+indices greater than \var{length} as errors.
+
+Returns 0 on success and -1 on error with no exception set (unless one
+of the indices was not \constant{None} and failed to be converted to
+an integer, in which case -1 is returned with an exception set).
+
+You probably do not want to use this function.  If you want to use
+slice objects in versions of Python prior to 2.3, you would probably
+do well to incorporate the source of \cfunction{PySlice_GetIndicesEx},
+suitably renamed, in the source of your extension.
+\end{cfuncdesc}
+
+\begin{cfuncdesc}{int}{PySlice_GetIndicesEx}{PySliceObject *slice, int length,
+                                             int *start, int *stop, int *step, 
+                                             int *slicelength}
+Usable replacement for \cfunction{PySlice_GetIndices}.  Retrieve the
+start, stop, and step indices from the slice object \var{slice}
+assuming a sequence of length \var{length}, and store the length of
+the slice in \var{slicelength}.  Out of bounds indices are clipped in
+a manner consistent with the handling of normal slices.
+
+Returns 0 on success and -1 on error with exception set.
+
+\versionadded{2.3}
 \end{cfuncdesc}
 
 

Doc/lib/libstdtypes.tex

@@ -433,6 +433,7 @@ equal to \var{x}, else \code{1}}{}
   \hline
   \lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(2)}
   \lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(2), (3)}
+  \lineiii{\var{s}[\var{i}:\var{j}:\var{k}]}{slice of \var{s} from \var{i} to \var{j} with step \var{k}}{(2), (4)}
   \hline
   \lineiii{len(\var{s})}{length of \var{s}}{}
   \lineiii{min(\var{s})}{smallest item of \var{s}}{}
@@ -446,6 +447,7 @@ equal to \var{x}, else \code{1}}{}
 \indexii{repetition}{operation}
 \indexii{subscript}{operation}
 \indexii{slice}{operation}
+\indexii{extended slice}{operation}
 \opindex{in}
 \opindex{not in}
 
@@ -492,6 +494,15 @@ Notes:
   \code{len(\var{s})}, use \code{len(\var{s})}.  If \var{i} is omitted,
   use \code{0}.  If \var{j} is omitted, use \code{len(\var{s})}.  If
   \var{i} is greater than or equal to \var{j}, the slice is empty.
+
+\item[(4)] The slice of \var{s} from \var{i} to \var{j} with step \var{k} 
+  is defined as the sequence of items with index \code{\var{x} =
+  \var{i} + \var{n}*\var{k}} such that \var{n} \code{>=} \code{0} and
+  \code{\var{i} <= \var{x} < \var{j}}.  If \var{i} or \var{j} is
+  greater than \code{len(\var{s})}, use \code{len(\var{s})}.  If
+  \var{i} or \var{j} are ommitted then they become ``end'' values
+  (which end depends on the sign of \var{k}).
+
 \end{description}
 
 
@@ -875,31 +886,36 @@ The following operations are defined on mutable sequence types (where
   	{slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{}
   \lineiii{del \var{s}[\var{i}:\var{j}]}
 	{same as \code{\var{s}[\var{i}:\var{j}] = []}}{}
+  \lineiii{\var{s}[\var{i}:\var{j}:\var{k}] = \var{t}}
+  	{the elements of \code{\var{s}[\var{i}:\var{j}:\var{k}]} are replaced by those of \var{t}}{(1)}
+  \lineiii{del \var{s}[\var{i}:\var{j}:\var{k}]}
+	{removes the elements of \code{\var{s}[\var{i}:\var{j}:\var{k}]} from the list}{}
   \lineiii{\var{s}.append(\var{x})}
-	{same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{(1)}
+	{same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{(2)}
   \lineiii{\var{s}.extend(\var{x})}
-        {same as \code{\var{s}[len(\var{s}):len(\var{s})] = \var{x}}}{(2)}
+        {same as \code{\var{s}[len(\var{s}):len(\var{s})] = \var{x}}}{(3)}
   \lineiii{\var{s}.count(\var{x})}
     {return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{}
   \lineiii{\var{s}.index(\var{x})}
-    {return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(3)}
+    {return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(4)}
   \lineiii{\var{s}.insert(\var{i}, \var{x})}
 	{same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}
-	  if \code{\var{i} >= 0}}{(4)}
+	  if \code{\var{i} >= 0}}{(5)}
   \lineiii{\var{s}.pop(\optional{\var{i}})}
-    {same as \code{\var{x} = \var{s}[\var{i}]; del \var{s}[\var{i}]; return \var{x}}}{(5)}
+    {same as \code{\var{x} = \var{s}[\var{i}]; del \var{s}[\var{i}]; return \var{x}}}{(6)}
   \lineiii{\var{s}.remove(\var{x})}
-	{same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(3)}
+	{same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(4)}
   \lineiii{\var{s}.reverse()}
-	{reverses the items of \var{s} in place}{(6)}
+	{reverses the items of \var{s} in place}{(7)}
   \lineiii{\var{s}.sort(\optional{\var{cmpfunc}})}
-	{sort the items of \var{s} in place}{(6), (7)}
+	{sort the items of \var{s} in place}{(7), (8)}
 \end{tableiii}
 \indexiv{operations on}{mutable}{sequence}{types}
 \indexiii{operations on}{sequence}{types}
 \indexiii{operations on}{list}{type}
 \indexii{subscript}{assignment}
 \indexii{slice}{assignment}
+\indexii{extended slice}{assignment}
 \stindex{del}
 \withsubitem{(list method)}{
   \ttindex{append()}\ttindex{extend()}\ttindex{count()}\ttindex{index()}
@@ -908,32 +924,35 @@ The following operations are defined on mutable sequence types (where
 \noindent
 Notes:
 \begin{description}
-\item[(1)] The C implementation of Python has historically accepted
+\item[(1)] \var{t} must have the same length as the slice it is 
+  replacing.
+
+\item[(2)] The C implementation of Python has historically accepted
   multiple parameters and implicitly joined them into a tuple; this
   no longer works in Python 2.0.  Use of this misfeature has been
   deprecated since Python 1.4.
 
-\item[(2)] Raises an exception when \var{x} is not a list object.  The
+\item[(3)] Raises an exception when \var{x} is not a list object.  The
   \method{extend()} method is experimental and not supported by
   mutable sequence types other than lists.
 
-\item[(3)] Raises \exception{ValueError} when \var{x} is not found in
+\item[(4)] Raises \exception{ValueError} when \var{x} is not found in
   \var{s}.
 
-\item[(4)] When a negative index is passed as the first parameter to
+\item[(5)] When a negative index is passed as the first parameter to
   the \method{insert()} method, the new element is prepended to the
   sequence.
 
-\item[(5)] The \method{pop()} method is only supported by the list and
+\item[(6)] The \method{pop()} method is only supported by the list and
   array types.  The optional argument \var{i} defaults to \code{-1},
   so that by default the last item is removed and returned.
 
-\item[(6)] The \method{sort()} and \method{reverse()} methods modify the
+\item[(7)] The \method{sort()} and \method{reverse()} methods modify the
   list in place for economy of space when sorting or reversing a large
   list.  To remind you that they operate by side effect, they don't return
   the sorted or reversed list.
 
-\item[(7)] The \method{sort()} method takes an optional argument
+\item[(8)] The \method{sort()} method takes an optional argument
   specifying a comparison function of two arguments (list items) which
   should return a negative, zero or positive number depending on whether
   the first argument is considered smaller than, equal to, or larger

Doc/ref/ref3.tex

@@ -252,6 +252,13 @@ sequence of the same type.  This implies that the index set is
 renumbered so that it starts at 0.
 \index{slicing}
 
+Some sequences also support ``extended slicing'' with a third ``step''
+parameter: \code{\var{a}[\var{i}:\var{j}:\var{k}]} selects all items
+of \var{a} with index \var{x} where \code{\var{x} = \var{i} +
+\var{n}*\var{k}}, \var{n} \code{>=} \code{0} and \var{i} \code{<=}
+\var{x} \code{<} \var{j}.
+\index{extended slicing}
+
 Sequences are distinguished according to their mutability:
 
 \begin{description}

Doc/whatsnew/whatsnew23.tex

@@ -336,6 +336,15 @@ strings \samp{True} and \samp{False} instead of \samp{1} and \samp{0}.
 
 \end{seealso}
 
+\section{Extended Slices\label{extended-slices}}
+
+Ever since Python 1.4 the slice syntax has supported a third
+``stride'' argument, but the builtin sequence types have not supported
+this feature (it was initially included at the behest of the
+developers of the Numerical Python package).  This changes with Python
+2.3.
+
+% XXX examples, etc.
 
 %======================================================================
 %\section{Other Language Changes}

Include/sliceobject.h

@@ -32,6 +32,9 @@ DL_IMPORT(PyObject *) PySlice_New(PyObject* start, PyObject* stop,
                                   PyObject* step);
 DL_IMPORT(int) PySlice_GetIndices(PySliceObject *r, int length,
                                   int *start, int *stop, int *step);
+DL_IMPORT(int) PySlice_GetIndicesEx(PySliceObject *r, int length,
+				    int *start, int *stop, 
+				    int *step, int *slicelength);
 
 #ifdef __cplusplus
 }

Lib/test/test_bool.py

@@ -219,7 +219,7 @@ veris(operator.isSequenceType(0), False)
 veris(operator.isSequenceType([]), True)
 veris(operator.contains([], 1), False)
 veris(operator.contains([1], 1), True)
-veris(operator.isMappingType([]), False)
+veris(operator.isMappingType(1), False)
 veris(operator.isMappingType({}), True)
 veris(operator.lt(0, 0), False)
 veris(operator.lt(0, 1), True)

Lib/test/test_types.py

@@ -188,6 +188,31 @@ else: raise TestFailed, 'in/not in string'
 x = 'x'*103
 if '%s!'%x != x+'!': raise TestFailed, 'nasty string formatting bug'
 
+#extended slices for strings
+a = '0123456789'
+vereq(a[::], a)
+vereq(a[::2], '02468')
+vereq(a[1::2], '13579')
+vereq(a[::-1],'9876543210')
+vereq(a[::-2], '97531')
+vereq(a[3::-2], '31')
+vereq(a[-100:100:], a)
+vereq(a[100:-100:-1], a[::-1])
+vereq(a[-100L:100L:2L], '02468')
+
+if have_unicode:
+    a = unicode('0123456789', 'ascii')
+    vereq(a[::], a)
+    vereq(a[::2], unicode('02468', 'ascii'))
+    vereq(a[1::2], unicode('13579', 'ascii'))
+    vereq(a[::-1], unicode('9876543210', 'ascii'))
+    vereq(a[::-2], unicode('97531', 'ascii'))
+    vereq(a[3::-2], unicode('31', 'ascii'))
+    vereq(a[-100:100:], a)
+    vereq(a[100:-100:-1], a[::-1])
+    vereq(a[-100L:100L:2L], unicode('02468', 'ascii'))
+    
+
 print '6.5.2 Tuples'
 if len(()) != 0: raise TestFailed, 'len(())'
 if len((1,)) != 1: raise TestFailed, 'len((1,))'
@@ -207,6 +232,19 @@ if x != (): raise TestFailed, 'tuple inplace add from () to () failed'
 x += (1,)
 if x != (1,): raise TestFailed, 'tuple resize from () failed'
 
+# extended slicing - subscript only for tuples
+a = (0,1,2,3,4)
+vereq(a[::], a)
+vereq(a[::2], (0,2,4))
+vereq(a[1::2], (1,3))
+vereq(a[::-1], (4,3,2,1,0))
+vereq(a[::-2], (4,2,0))
+vereq(a[3::-2], (3,1))
+vereq(a[-100:100:], a)
+vereq(a[100:-100:-1], a[::-1])
+vereq(a[-100L:100L:2L], (0,2,4))
+
+
 print '6.5.3 Lists'
 if len([]) != 0: raise TestFailed, 'len([])'
 if len([1,]) != 1: raise TestFailed, 'len([1,])'
@@ -322,6 +360,40 @@ if a[ -pow(2,128L): 3 ] != [0,1,2]:
 if a[ 3: pow(2,145L) ] != [3,4]:
     raise TestFailed, "list slicing with too-large long integer"
 
+
+# extended slicing
+
+#  subscript
+a = [0,1,2,3,4]
+vereq(a[::], a)
+vereq(a[::2], [0,2,4])
+vereq(a[1::2], [1,3])
+vereq(a[::-1], [4,3,2,1,0])
+vereq(a[::-2], [4,2,0])
+vereq(a[3::-2], [3,1])
+vereq(a[-100:100:], a)
+vereq(a[100:-100:-1], a[::-1])
+vereq(a[-100L:100L:2L], [0,2,4])
+#  deletion
+del a[::2]
+vereq(a, [1,3])
+a = range(5)
+del a[1::2]
+vereq(a, [0,2,4])
+a = range(5)
+del a[1::-2]
+vereq(a, [0,2,3,4])
+#  assignment
+a = range(10)
+a[::2] = [-1]*5
+vereq(a, [-1, 1, -1, 3, -1, 5, -1, 7, -1, 9])
+a = range(10)
+a[::-4] = [10]*3
+vereq(a, [0, 10, 2, 3, 4, 10, 6, 7, 8 ,10])
+a = range(4)
+a[::-1] = a
+vereq(a, [3, 2, 1, 0])
+
 print '6.6 Mappings == Dictionaries'
 d = {}
 if d.keys() != []: raise TestFailed, '{}.keys()'

Misc/NEWS

@@ -6,6 +6,10 @@ Type/class unification and new-style classes
 
 Core and builtins
 
+- Most builtin sequences now support "extended slices", i.e. slices
+  with a third "stride" parameter.  For example, "range(10)[1:6:2]"
+  evaluates to [1, 3, 5].
+
 - Cycles going through the __class__ link of a new-style instance are
   now detected by the garbage collector.
 

Objects/listobject.c

@@ -1684,6 +1684,192 @@ static char list_doc[] =
 
 staticforward PyObject * list_iter(PyObject *seq);
 
+static PyObject*
+list_subscript(PyListObject* self, PyObject* item)
+{
+	if (PyInt_Check(item)) {
+		long i = PyInt_AS_LONG(item);
+		if (i < 0)
+			i += PyList_GET_SIZE(self);
+		return list_item(self, i);
+	}
+	else if (PyLong_Check(item)) {
+		long i = PyLong_AsLong(item);
+		if (i == -1 && PyErr_Occurred())
+			return NULL;
+		if (i < 0)
+			i += PyList_GET_SIZE(self);
+		return list_item(self, i);
+	}
+	else if (PySlice_Check(item)) {
+		int start, stop, step, slicelength, cur, i;
+		PyObject* result;
+		PyObject* it;
+
+		if (PySlice_GetIndicesEx((PySliceObject*)item, self->ob_size,
+				 &start, &stop, &step, &slicelength) < 0) {
+			return NULL;
+		}
+
+		if (slicelength <= 0) {
+			return PyList_New(0);
+		}
+		else {
+			result = PyList_New(slicelength);
+			if (!result) return NULL;
+
+			for (cur = start, i = 0; i < slicelength; 
+			     cur += step, i++) {
+				it = PyList_GET_ITEM(self, cur);
+				Py_INCREF(it);
+				PyList_SET_ITEM(result, i, it);
+			}
+			
+			return result;
+		}
+	}
+	else {
+		PyErr_SetString(PyExc_TypeError,
+				"list indices must be integers");
+		return NULL;
+	}
+}
+
+static int 
+list_ass_subscript(PyListObject* self, PyObject* item, PyObject* value)
+{
+	if (PyInt_Check(item)) {
+		long i = PyInt_AS_LONG(item);
+		if (i < 0)
+			i += PyList_GET_SIZE(self);
+		return list_ass_item(self, i, value);
+	}
+	else if (PyLong_Check(item)) {
+		long i = PyLong_AsLong(item);
+		if (i == -1 && PyErr_Occurred())
+			return -1;
+		if (i < 0)
+			i += PyList_GET_SIZE(self);
+		return list_ass_item(self, i, value);
+	}
+	else if (PySlice_Check(item)) {
+		int start, stop, step, slicelength;
+
+		if (PySlice_GetIndicesEx((PySliceObject*)item, self->ob_size,
+				 &start, &stop, &step, &slicelength) < 0) {
+			return -1;
+		}
+
+		if (value == NULL) {
+			/* delete slice */
+			PyObject **garbage, **item;
+			int cur, i, j;
+			
+			if (slicelength <= 0)
+				return 0;
+
+			if (step < 0) {
+				stop = start + 1;
+				start = stop + step*(slicelength - 1) - 1;
+				step = -step;
+			}
+
+			garbage = (PyObject**)
+				PyMem_MALLOC(slicelength*sizeof(PyObject*));
+			
+			/* drawing pictures might help 
+			   understand these for loops */
+			for (cur = start, i = 0; cur < stop; cur += step, i++) {
+				garbage[i] = PyList_GET_ITEM(self, cur);
+
+				for (j = 0; j < step; j++) {
+					PyList_SET_ITEM(self, cur + j - i, 
+						PyList_GET_ITEM(self, cur + j + 1));
+				}
+			}
+			for (cur = start + slicelength*step + 1; 
+			     cur < self->ob_size; cur++) {
+				PyList_SET_ITEM(self, cur - slicelength,
+						PyList_GET_ITEM(self, cur));
+			}
+			self->ob_size -= slicelength;
+			item = self->ob_item;
+			NRESIZE(item, PyObject*, self->ob_size);
+			self->ob_item = item;
+
+			for (i = 0; i < slicelength; i++) {
+				Py_DECREF(garbage[i]);
+			}
+			PyMem_FREE(garbage);
+
+			return 0;
+		}
+		else {
+			/* assign slice */
+			PyObject **garbage, *ins;
+			int cur, i;
+
+			if (!PyList_Check(value)) {
+				PyErr_Format(PyExc_TypeError,
+			     "must assign list (not \"%.200s\") to slice",
+					     value->ob_type->tp_name);
+				return -1;
+			}
+
+			if (PyList_GET_SIZE(value) != slicelength) {
+				PyErr_Format(PyExc_ValueError,
+            "attempt to assign list of size %d to extended slice of size %d",
+					     PyList_Size(value), slicelength);
+				return -1;
+			}
+
+			if (!slicelength)
+				return 0;
+
+			/* protect against a[::-1] = a */
+			if (self == (PyListObject*)value) { 
+				value = list_slice((PyListObject*)value, 0,
+						   PyList_GET_SIZE(value));
+			} 
+			else {
+				Py_INCREF(value);
+			}
+
+			garbage = (PyObject**)
+				PyMem_MALLOC(slicelength*sizeof(PyObject*));
+			
+			for (cur = start, i = 0; i < slicelength; 
+			     cur += step, i++) {
+				garbage[i] = PyList_GET_ITEM(self, cur);
+				
+				ins = PyList_GET_ITEM(value, i);
+				Py_INCREF(ins);
+				PyList_SET_ITEM(self, cur, ins);
+			}
+
+			for (i = 0; i < slicelength; i++) {
+				Py_DECREF(garbage[i]);
+			}
+			
+			PyMem_FREE(garbage);
+			Py_DECREF(value);
+			
+			return 0;
+		}
+	} 
+	else {
+		PyErr_SetString(PyExc_TypeError, 
+				"list indices must be integers");
+		return -1;
+	}
+}
+
+static PyMappingMethods list_as_mapping = {
+	(inquiry)list_length,
+	(binaryfunc)list_subscript,
+	(objobjargproc)list_ass_subscript
+};
+
 PyTypeObject PyList_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)
 	0,
@@ -1698,7 +1884,7 @@ PyTypeObject PyList_Type = {
 	(reprfunc)list_repr,			/* tp_repr */
 	0,					/* tp_as_number */
 	&list_as_sequence,			/* tp_as_sequence */
-	0,					/* tp_as_mapping */
+	&list_as_mapping,			/* tp_as_mapping */
 	list_nohash,				/* tp_hash */
 	0,					/* tp_call */
 	0,					/* tp_str */

Objects/sliceobject.c

@@ -109,6 +109,59 @@ PySlice_GetIndices(PySliceObject *r, int length,
 	return 0;
 }
 
+int
+PySlice_GetIndicesEx(PySliceObject *r, int length,
+		     int *start, int *stop, int *step, int *slicelength)
+{
+	/* this is harder to get right than you might think */
+	int defstart, defstop;
+
+	if (r->step == Py_None) {
+		*step = 1;
+	} else {
+		*step = PyInt_AsLong(r->step);
+		if (*step == -1 && PyErr_Occurred()) {
+			return -1;
+		}
+		else if (*step == 0) {
+			PyErr_SetString(PyExc_ValueError,
+					"slice step cannot be zero");
+			return -1;
+		}
+	}
+
+	defstart = *step < 0 ? length-1 : 0;
+	defstop = *step < 0 ? -1 : length;
+
+	if (r->start == Py_None) {
+		*start = defstart;
+	} else {
+		if (!_PyEval_SliceIndex(r->start, start)) return -1;
+		if (*start < 0) *start += length;
+		if (*start < 0) *start = (*step < 0) ? -1 : 0;
+		if (*start >= length) 
+			*start = (*step < 0) ? length - 1 : length;
+	}
+
+	if (r->stop == Py_None) {
+		*stop = defstop;
+	} else {
+		if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
+		if (*stop < 0) *stop += length;
+		if (*stop < 0) *stop = -1;
+		if (*stop > length) *stop = length;
+	}
+
+	if (*step < 0) {
+		*slicelength = (*stop-*start+1)/(*step)+1;
+	} else {
+		*slicelength = (*stop-*start-1)/(*step)+1;
+	}
+	if (*slicelength < 0) *slicelength = 0;
+
+	return 0;
+}
+
 static void
 slice_dealloc(PySliceObject *r)
 {

Objects/stringobject.c

@@ -940,6 +940,60 @@ string_hash(PyStringObject *a)
 	return x;
 }
 
+static PyObject*
+string_subscript(PyStringObject* self, PyObject* item)
+{
+	if (PyInt_Check(item)) {
+		long i = PyInt_AS_LONG(item);
+		if (i < 0)
+			i += PyString_GET_SIZE(self);
+		return string_item(self,i);
+	}
+	else if (PyLong_Check(item)) {
+		long i = PyLong_AsLong(item);
+		if (i == -1 && PyErr_Occurred())
+			return NULL;
+		if (i < 0)
+			i += PyString_GET_SIZE(self);
+		return string_item(self,i);
+	}
+	else if (PySlice_Check(item)) {
+		int start, stop, step, slicelength, cur, i;
+		char* source_buf;
+		char* result_buf;
+		PyObject* result;
+
+		if (PySlice_GetIndicesEx((PySliceObject*)item, 
+				 PyString_GET_SIZE(self),
+				 &start, &stop, &step, &slicelength) < 0) {
+			return NULL;
+		}
+
+		if (slicelength <= 0) {
+			return PyString_FromStringAndSize("", 0);
+		}
+		else {
+			source_buf = PyString_AsString((PyObject*)self);
+			result_buf = PyMem_Malloc(slicelength);
+
+			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, 
+				"string indices must be integers");
+		return NULL;
+	}
+}
+
 static int
 string_buffer_getreadbuf(PyStringObject *self, int index, const void **ptr)
 {
@@ -991,6 +1045,12 @@ static PySequenceMethods string_as_sequence = {
 	(objobjproc)string_contains /*sq_contains*/
 };
 
+static PyMappingMethods string_as_mapping = {
+	(inquiry)string_length,
+	(binaryfunc)string_subscript,
+	0,
+};
+
 static PyBufferProcs string_as_buffer = {
 	(getreadbufferproc)string_buffer_getreadbuf,
 	(getwritebufferproc)string_buffer_getwritebuf,
@@ -2929,7 +2989,7 @@ PyTypeObject PyString_Type = {
 	(reprfunc)string_repr, 			/* tp_repr */
 	0,					/* tp_as_number */
 	&string_as_sequence,			/* tp_as_sequence */
-	0,					/* tp_as_mapping */
+	&string_as_mapping,			/* tp_as_mapping */
 	(hashfunc)string_hash, 			/* tp_hash */
 	0,					/* tp_call */
 	(reprfunc)string_str,			/* tp_str */
@@ -3349,7 +3409,7 @@ PyString_Format(PyObject *format, PyObject *args)
 		arglen = -1;
 		argidx = -2;
 	}
-	if (args->ob_type->tp_as_mapping)
+	if (args->ob_type->tp_as_mapping && !PyTuple_Check(args))
 		dict = args;
 	while (--fmtcnt >= 0) {
 		if (*fmt != '%') {

Objects/tupleobject.c

@@ -541,6 +541,63 @@ static PySequenceMethods tuple_as_sequence = {
 	(objobjproc)tuplecontains,		/* sq_contains */
 };
 
+static PyObject*
+tuplesubscript(PyTupleObject* self, PyObject* item)
+{
+	if (PyInt_Check(item)) {
+		long i = PyInt_AS_LONG(item);
+		if (i < 0)
+			i += PyTuple_GET_SIZE(self);
+		return tupleitem(self, i);
+	}
+	else if (PyLong_Check(item)) {
+		long i = PyLong_AsLong(item);
+		if (i == -1 && PyErr_Occurred())
+			return NULL;
+		if (i < 0)
+			i += PyTuple_GET_SIZE(self);
+		return tupleitem(self, i);
+	}
+	else if (PySlice_Check(item)) {
+		int start, stop, step, slicelength, cur, i;
+		PyObject* result;
+		PyObject* it;
+
+		if (PySlice_GetIndicesEx((PySliceObject*)item,
+				 PyTuple_GET_SIZE(self),
+				 &start, &stop, &step, &slicelength) < 0) {
+			return NULL;
+		}
+
+		if (slicelength <= 0) {
+			return PyTuple_New(0);
+		}
+		else {
+			result = PyTuple_New(slicelength);
+
+			for (cur = start, i = 0; i < slicelength; 
+			     cur += step, i++) {
+				it = PyTuple_GET_ITEM(self, cur);
+				Py_INCREF(it);
+				PyTuple_SET_ITEM(result, i, it);
+			}
+			
+			return result;
+		}
+	}
+	else {
+		PyErr_SetString(PyExc_TypeError, 
+				"tuple indices must be integers");
+		return NULL;
+	}
+}
+
+static PyMappingMethods tuple_as_mapping = {
+	(inquiry)tuplelength,
+	(binaryfunc)tuplesubscript,
+	0
+};
+
 PyTypeObject PyTuple_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)
 	0,
@@ -555,7 +612,7 @@ PyTypeObject PyTuple_Type = {
 	(reprfunc)tuplerepr,			/* tp_repr */
 	0,					/* tp_as_number */
 	&tuple_as_sequence,			/* tp_as_sequence */
-	0,					/* tp_as_mapping */
+	&tuple_as_mapping,			/* tp_as_mapping */
 	(hashfunc)tuplehash,			/* tp_hash */
 	0,					/* tp_call */
 	0,					/* tp_str */

Objects/unicodeobject.c

@@ -5061,6 +5061,58 @@ static PySequenceMethods unicode_as_sequence = {
     (objobjproc)PyUnicode_Contains, 	/*sq_contains*/
 };
 
+static PyObject*
+unicode_subscript(PyUnicodeObject* self, PyObject* item)
+{
+    if (PyInt_Check(item)) {
+        long i = PyInt_AS_LONG(item);
+        if (i < 0)
+            i += PyString_GET_SIZE(self);
+        return unicode_getitem(self, i);
+    } else if (PyLong_Check(item)) {
+        long i = PyLong_AsLong(item);
+        if (i == -1 && PyErr_Occurred())
+            return NULL;
+        if (i < 0)
+            i += PyString_GET_SIZE(self);
+        return unicode_getitem(self, i);
+    } else if (PySlice_Check(item)) {
+        int start, stop, step, slicelength, cur, i;
+        Py_UNICODE* source_buf;
+        Py_UNICODE* result_buf;
+        PyObject* result;
+
+        if (PySlice_GetIndicesEx((PySliceObject*)item, PyString_GET_SIZE(self),
+				 &start, &stop, &step, &slicelength) < 0) {
+            return NULL;
+        }
+
+        if (slicelength <= 0) {
+            return PyUnicode_FromUnicode(NULL, 0);
+        } else {
+            source_buf = PyUnicode_AS_UNICODE((PyObject*)self);
+            result_buf = PyMem_MALLOC(slicelength*sizeof(Py_UNICODE));
+
+            for (cur = start, i = 0; i < slicelength; cur += step, i++) {
+                result_buf[i] = source_buf[cur];
+            }
+            
+            result = PyUnicode_FromUnicode(result_buf, slicelength);
+            PyMem_FREE(result_buf);
+            return result;
+        }
+    } else {
+        PyErr_SetString(PyExc_TypeError, "string indices must be integers");
+        return NULL;
+    }
+}
+
+static PyMappingMethods unicode_as_mapping = {
+    (inquiry)unicode_length,		/* mp_length */
+    (binaryfunc)unicode_subscript,	/* mp_subscript */
+    (objobjargproc)0,			/* mp_ass_subscript */
+};
+
 static int
 unicode_buffer_getreadbuf(PyUnicodeObject *self,
 			  int index,
@@ -5355,7 +5407,7 @@ PyObject *PyUnicode_Format(PyObject *format,
 	arglen = -1;
 	argidx = -2;
     }
-    if (args->ob_type->tp_as_mapping)
+    if (args->ob_type->tp_as_mapping && !PyTuple_Check(args))
 	dict = args;
 
     while (--fmtcnt >= 0) {
@@ -5817,7 +5869,7 @@ PyTypeObject PyUnicode_Type = {
     (reprfunc) unicode_repr, 		/* tp_repr */
     0, 					/* tp_as_number */
     &unicode_as_sequence, 		/* tp_as_sequence */
-    0, 					/* tp_as_mapping */
+    &unicode_as_mapping, 		/* tp_as_mapping */
     (hashfunc) unicode_hash, 		/* tp_hash*/
     0, 					/* tp_call*/
     (reprfunc) unicode_str,	 	/* tp_str */