bpo-33176 Add readonly mode to memoryviews

Doc/library/stdtypes.rst

@@ -3624,13 +3624,15 @@ copying.
 
       .. versionadded:: 3.2
 
-   .. method:: cast(format[, shape])
+   .. method:: cast(format[, shape, readonly])
 
       Cast a memoryview to a new format or shape. *shape* defaults to
       ``[byte_length//new_itemsize]``, which means that the result view
       will be one-dimensional. The return value is a new memoryview, but
       the buffer itself is not copied. Supported casts are 1D -> C-:term:`contiguous`
-      and C-contiguous -> 1D.
+      and C-contiguous -> 1D. If *readonly* is ``True`` the result view will
+      not be writable. Triying to cast a view with a readonly underlying buffer to
+      a writable view will raise a :class:`ValueError`.
 
       The destination format is restricted to a single element native format in
       :mod:`struct` syntax. One of the formats must be a byte format

Lib/test/test_memoryview.py

@@ -71,6 +71,36 @@ def setitem(value):
         m = None
         self.assertEqual(sys.getrefcount(b), oldrefcount)
 
+    def test_setitem_readonly_and_writable_buffer(self):
+        if not self.rw_type:
+            self.skipTest("no writable type to test")
+        b = self.rw_type(self._source)
+        oldrefcount = sys.getrefcount(b)
+        m = self._view(b)
+        readonly_m = m.cast("B", readonly=True)
+        def setitem(value):
+            readonly_m[0] = value
+        self.assertRaises(TypeError, setitem, 12)
+        m = None
+        readonly_m = None
+        self.assertEqual(sys.getrefcount(b), oldrefcount)
+
+    def test_setitem_readonly_and_readonly_buffer(self):
+        if not self.ro_type:
+            self.skipTest("no writable type to test")
+        tp = self.ro_type
+        b = self.ro_type(self._source)
+        oldrefcount = sys.getrefcount(b)
+        m = self._view(b)
+        self.assertRaises(TypeError, lambda: m.cast("B", readonly=False))
+        readonly_m = m.cast("B", readonly=True)
+        def setitem(value):
+            readonly_m[0] = value
+        self.assertRaises(TypeError, setitem, 12)
+        m = None
+        readonly_m = None
+        self.assertEqual(sys.getrefcount(b), oldrefcount)
+
     def test_setitem_writable(self):
         if not self.rw_type:
             self.skipTest("no writable type to test")

Misc/NEWS.d/next/Core and Builtins/2018-03-30-18-27-56.bpo-33176.PS493k.rst

@@ -0,0 +1 @@
+Add readonly flag to memoryview.cast to create readonly memoryviews.

Objects/memoryobject.c

@@ -1338,16 +1338,17 @@ zero_in_shape(PyMemoryViewObject *mv)
 static PyObject *
 memory_cast(PyMemoryViewObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"format", "shape", NULL};
+    static char *kwlist[] = {"format", "shape", "readonly", NULL};
     PyMemoryViewObject *mv = NULL;
     PyObject *shape = NULL;
+    int readonly = self->view.readonly;
     PyObject *format;
     Py_ssize_t ndim = 1;
 
     CHECK_RELEASED(self);
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O", kwlist,
-                                     &format, &shape)) {
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O$p", kwlist,
+                                     &format, &shape, &readonly)) {
         return NULL;
     }
     if (!PyUnicode_Check(format)) {
@@ -1365,6 +1366,13 @@ memory_cast(PyMemoryViewObject *self, PyObject *args, PyObject *kwds)
             "memoryview: cannot cast view with zeros in shape or strides");
         return NULL;
     }
+
+    if (self->view.readonly && !readonly){
+        PyErr_SetString(PyExc_TypeError,
+        "memoryview: cannot cast readonly buffer to a writable buffer.");
+        return NULL;
+    }
+
     if (shape) {
         CHECK_LIST_OR_TUPLE(shape)
         ndim = PySequence_Fast_GET_SIZE(shape);
@@ -1391,6 +1399,8 @@ memory_cast(PyMemoryViewObject *self, PyObject *args, PyObject *kwds)
     if (shape && cast_to_ND(mv, shape, (int)ndim) < 0)
         goto error;
 
+    mv->view.readonly = readonly;
+
     return (PyObject *)mv;
 
 error:
@@ -3058,7 +3068,7 @@ PyDoc_STRVAR(memory_tolist_doc,
 \n\
 Return the data in the buffer as a list of elements.");
 PyDoc_STRVAR(memory_cast_doc,
-"cast($self, /, format, *, shape)\n--\n\
+"cast($self, /, format, *, shape, readonly)\n--\n\
 \n\
 Cast a memoryview to a new format or shape.");