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 @@ \subsection{Slice Objects \label{slice-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 @@ \subsection{Sequence Types \label{typesseq}}
   \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 @@ \subsection{Sequence Types \label{typesseq}}
 \indexii{repetition}{operation}
 \indexii{subscript}{operation}
 \indexii{slice}{operation}
+\indexii{extended slice}{operation}
 \opindex{in}
 \opindex{not in}
 
@@ -492,6 +494,15 @@ \subsection{Sequence Types \label{typesseq}}
   \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 @@ \subsubsection{Mutable Sequence Types \label{typesseq-mutable}}
   	{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 @@ \subsubsection{Mutable Sequence Types \label{typesseq-mutable}}
 \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 @@ \section{The standard type hierarchy\label{types}}
 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 @@ \section{PEP 285: The \class{bool} Type\label{section-bool}}
 
 \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 @@ class C(bool):
 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 @@ class C: pass
 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 @@ class C: pass
 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 @@ def myComparison(x,y):
 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.