# HG changeset patch
# Parent 19b2c54e5f093f6a345aa82b54c5295688c22790

diff -r 19b2c54e5f09 Include/sliceobject.h
--- a/Include/sliceobject.h	Wed Nov 12 10:23:44 2014 -0500
+++ b/Include/sliceobject.h	Sun Nov 16 09:14:02 2014 +0100
@@ -22,6 +22,8 @@
 typedef struct {
     PyObject_HEAD
     PyObject *start, *stop, *step;	/* not NULL */
+    Py_ssize_t istart, istop, istep;
+    int flags;
 } PySliceObject;
 #endif
 
@@ -33,6 +35,7 @@
 PyAPI_FUNC(PyObject *) PySlice_New(PyObject* start, PyObject* stop,
                                   PyObject* step);
 #ifndef Py_LIMITED_API
+PyAPI_FUNC(PyObject *) PySlice_FromIndices(Py_ssize_t start, Py_ssize_t stop, Py_ssize_t step);
 PyAPI_FUNC(PyObject *) _PySlice_FromIndices(Py_ssize_t start, Py_ssize_t stop);
 PyAPI_FUNC(int) _PySlice_GetLongIndices(PySliceObject *self, PyObject *length,
                                  PyObject **start_ptr, PyObject **stop_ptr,
diff -r 19b2c54e5f09 Objects/sliceobject.c
--- a/Objects/sliceobject.c	Wed Nov 12 10:23:44 2014 -0500
+++ b/Objects/sliceobject.c	Sun Nov 16 09:14:02 2014 +0100
@@ -93,6 +93,10 @@
 
 /* Slice object implementation */
 
+#define ISTART_OK 1
+#define ISTOP_OK 2
+#define ISTEP_OK 4
+
 /* Using a cache is very effective since typically only a single slice is
  * created and then deleted again
  */
@@ -114,6 +118,7 @@
 PySlice_New(PyObject *start, PyObject *stop, PyObject *step)
 {
     PySliceObject *obj;
+    int flags = 0;
     if (slice_cache != NULL) {
         obj = slice_cache;
         slice_cache = NULL;
@@ -124,9 +129,15 @@
             return NULL;
     }
 
-    if (step == NULL) step = Py_None;
+    if (step == NULL || step == Py_None) {
+        step = Py_None;
+        flags |= ISTEP_OK;
+    }
     Py_INCREF(step);
-    if (start == NULL) start = Py_None;
+    if (start == NULL || start == Py_None) {
+        start = Py_None;
+        flags |= ISTART_OK;
+    }
     Py_INCREF(start);
     if (stop == NULL) stop = Py_None;
     Py_INCREF(stop);
@@ -134,6 +145,9 @@
     obj->step = step;
     obj->start = start;
     obj->stop = stop;
+    obj->istart = 0;
+    obj->istep = 1;
+    obj->flags = flags;
 
     return (PyObject *) obj;
 }
@@ -141,7 +155,20 @@
 PyObject *
 _PySlice_FromIndices(Py_ssize_t istart, Py_ssize_t istop)
 {
-    PyObject *start, *end, *slice;
+    return PySlice_FromIndices(istart, istop, 1);
+}
+
+PyObject *
+PySlice_FromIndices(Py_ssize_t istart, Py_ssize_t istop, Py_ssize_t istep)
+{
+    PySliceObject *slobj;
+    PyObject *start, *end, *step, *slice;
+    if (istep == 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "slice step cannot be zero");
+        return NULL;
+    }
+
     start = PyLong_FromSsize_t(istart);
     if (!start)
         return NULL;
@@ -150,10 +177,28 @@
         Py_DECREF(start);
         return NULL;
     }
+    if (istep == 1) {
+        step = Py_None;
+        Py_INCREF(step);
+    }
+    else {
+        step = PyLong_FromSsize_t(istep);
+	if (!step) {
+	  Py_DECREF(start);
+	  Py_DECREF(end);
+	  return NULL;
+	}
+    }
 
-    slice = PySlice_New(start, end, NULL);
+    slice = PySlice_New(start, end, step);
     Py_DECREF(start);
     Py_DECREF(end);
+    Py_DECREF(step);
+    slobj = (PySliceObject*) slice;
+    slobj->istart = istart;
+    slobj->istop = istop;
+    slobj->istep = istep;
+    slobj->flags = ISTART_OK | ISTOP_OK | ISTEP_OK;
     return slice;
 }
 
@@ -203,7 +248,15 @@
         *step = 1;
     }
     else {
-        if (!_PyEval_SliceIndex(r->step, step)) return -1;
+        if (r->flags & ISTEP_OK) {
+#ifdef Py_DEBUG
+            if (!_PyEval_SliceIndex(r->step, step)) return -1;
+            assert(*step == r->istep);
+#endif
+            *step = r->istep;
+        } else {
+            if (!_PyEval_SliceIndex(r->step, step)) return -1;
+        }
         if (*step == 0) {
             PyErr_SetString(PyExc_ValueError,
                             "slice step cannot be zero");
@@ -225,7 +278,15 @@
         *start = defstart;
     }
     else {
-        if (!_PyEval_SliceIndex(r->start, start)) return -1;
+        if (r->flags & ISTART_OK) {
+#ifdef Py_DEBUG
+            if (!_PyEval_SliceIndex(r->start, start)) return -1;
+            assert(*start == r->istart);
+#endif
+            *start = r->istart;
+        } 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)
@@ -236,7 +297,15 @@
         *stop = defstop;
     }
     else {
-        if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
+        if (r->flags & ISTOP_OK) {
+#ifdef Py_DEBUG
+            if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
+            assert(*stop == r->istop);
+#endif
+            *stop = r->istop;
+        } else {
+            if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
+        }
         if (*stop < 0) *stop += length;
         if (*stop < 0) *stop = (*step < 0) ? -1 : 0;
         if (*stop >= length)