numpy · charris · May 25, 2019 · May 3, 2019
diff --git a/doc/release/1.16.4-notes.rst b/doc/release/1.16.4-notes.rst
@@ -22,7 +22,15 @@ New functions
 
 New deprecations
 ================
-
+Writeable flag of C-API wrapped arrays
+--------------------------------------
+When an array is created from the C-API to wrap a pointer to data, the only
+indication we have of the read-write nature of the data is the ``writeable``
+flag set during creation. It is dangerous to force the flag to writeable.  In
+the future it will not be possible to switch the writeable flag to ``True``
+from python.  This deprecation should not affect many users since arrays
+created in such a manner are very rare in practice and only available through
+the NumPy C-API.
 
 Expired deprecations
 ====================

diff --git a/numpy/core/src/multiarray/_multiarray_tests.c.src b/numpy/core/src/multiarray/_multiarray_tests.c.src
@@ -847,6 +847,29 @@ get_buffer_info(PyObject *NPY_UNUSED(self), PyObject *args)
 
 #undef GET_PYBUF_FLAG
 
+/*
+ * Return a new array object wrapping existing C-allocated (dummy) data.
+ * Such an array does not own its data (must not free it), but because it
+ * wraps C data, it also has no base object. Used to test arr.flags.writeable
+ * setting behaviour.
+ */
+static PyObject*
+get_c_wrapping_array(PyObject* NPY_UNUSED(self), PyObject* arg)
+{
+    int writeable, flags;
+    npy_intp zero = 0;
+
+    writeable = PyObject_IsTrue(arg);
+    if (error_converting(writeable)) {
+        return NULL;
+    }
+
+    flags = writeable ? NPY_ARRAY_WRITEABLE : 0;
+    /* Create an empty array (which points to a random place) */
+    return PyArray_NewFromDescr(&PyArray_Type, PyArray_DescrFromType(NPY_INTP),
+                                1, &zero, NULL, &zero, flags, NULL);
+}
+
 
 /*
  * Test C-api level item getting.
@@ -1932,6 +1955,9 @@ static PyMethodDef Multiarray_TestsMethods[] = {
     {"get_buffer_info",
         get_buffer_info,
         METH_VARARGS, NULL},
+    {"get_c_wrapping_array",
+        get_c_wrapping_array,
+        METH_O, NULL},
     {"array_indexing",
         array_indexing,
         METH_VARARGS, NULL},

diff --git a/numpy/core/src/multiarray/common.c b/numpy/core/src/multiarray/common.c
@@ -598,31 +598,46 @@ _zerofill(PyArrayObject *ret)
 NPY_NO_EXPORT npy_bool
 _IsWriteable(PyArrayObject *ap)
 {
-    PyObject *base=PyArray_BASE(ap);
+    PyObject *base = PyArray_BASE(ap);
 #if defined(NPY_PY3K)
     Py_buffer view;
 #else
     void *dummy;
     Py_ssize_t n;
 #endif
 
-    /* If we own our own data, then no-problem */
-    if ((base == NULL) || (PyArray_FLAGS(ap) & NPY_ARRAY_OWNDATA)) {
+    /*
+     * C-data wrapping arrays may not own their data while not having a base;
+     * WRITEBACKIFCOPY arrays have a base, but do own their data.
+     */
+    if (base == NULL || PyArray_CHKFLAGS(ap, NPY_ARRAY_OWNDATA)) {
+        /*
+         * This is somewhat unsafe for directly wrapped non-writable C-arrays,
+         * which do not know whether the memory area is writable or not and
+         * do not own their data (but have no base).
+         * It would be better if this returned PyArray_ISWRITEABLE(ap).
+         * Since it is hard to deprecate, this is deprecated only on the Python
+         * side, but not on in PyArray_UpdateFlags.
+         */
         return NPY_TRUE;
     }
+
     /*
-     * Get to the final base object
-     * If it is a writeable array, then return TRUE
-     * If we can find an array object
-     * or a writeable buffer object as the final base object
+     * Get to the final base object.
+     * If it is a writeable array, then return True if we can
+     * find an array object or a writeable buffer object as
+     * the final base object.
      */
+    while (PyArray_Check(base)) {
+        ap = (PyArrayObject *)base;
+        base = PyArray_BASE(ap);
 
-    while(PyArray_Check(base)) {
-        if (PyArray_CHKFLAGS((PyArrayObject *)base, NPY_ARRAY_OWNDATA)) {
-            return (npy_bool) (PyArray_ISWRITEABLE((PyArrayObject *)base));
+        if (base == NULL || PyArray_CHKFLAGS(ap, NPY_ARRAY_OWNDATA)) {
+            return (npy_bool) PyArray_ISWRITEABLE(ap);
         }
-        base = PyArray_BASE((PyArrayObject *)base);
+        assert(!PyArray_CHKFLAGS(ap, NPY_ARRAY_OWNDATA));
     }
+
 #if defined(NPY_PY3K)
     if (PyObject_GetBuffer(base, &view, PyBUF_WRITABLE|PyBUF_SIMPLE) < 0) {
         PyErr_Clear();

diff --git a/numpy/core/src/multiarray/methods.c b/numpy/core/src/multiarray/methods.c
@@ -2546,6 +2546,22 @@ array_setflags(PyArrayObject *self, PyObject *args, PyObject *kwds)
     if (write_flag != Py_None) {
         if (PyObject_IsTrue(write_flag)) {
             if (_IsWriteable(self)) {
+                /*
+                 * _IsWritable (and PyArray_UpdateFlags) allows flipping this,
+                 * although the C-Api user who created the array may have
+                 * chosen to make it non-writable for a good reason, so
+                 * deprecate.
+                 */
+                if ((PyArray_BASE(self) == NULL) &&
+                            !PyArray_CHKFLAGS(self, NPY_ARRAY_OWNDATA) &&
+                            !PyArray_CHKFLAGS(self, NPY_ARRAY_WRITEABLE)) {
+                    /* 2017-05-03, NumPy 1.17.0 */
+                    if (DEPRECATE("making a non-writeable array writeable "
+                                  "is deprecated for arrays without a base "
+                                  "which do not own their data.") < 0) {
+                        return NULL;
+                    }
+                }
                 PyArray_ENABLEFLAGS(self, NPY_ARRAY_WRITEABLE);
             }
             else {

diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
@@ -142,6 +142,47 @@ def test_writeable_pickle(self):
             assert_(vals.flags.writeable)
             assert_(isinstance(vals.base, bytes))
 
+    def test_writeable_from_c_data(self):
+        # Test that the writeable flag can be changed for an array wrapping
+        # low level C-data, but not owning its data.
+        # Also see that this is deprecated to change from python.
+        from numpy.core._multiarray_tests import get_c_wrapping_array
+
+        arr_writeable = get_c_wrapping_array(True)
+        assert not arr_writeable.flags.owndata
+        assert arr_writeable.flags.writeable
+        view = arr_writeable[...]
+
+        # Toggling the writeable flag works on the view:
+        view.flags.writeable = False
+        assert not view.flags.writeable
+        view.flags.writeable = True
+        assert view.flags.writeable
+        # Flag can be unset on the arr_writeable:
+        arr_writeable.flags.writeable = False
+
+        arr_readonly = get_c_wrapping_array(False)
+        assert not arr_readonly.flags.owndata
+        assert not arr_readonly.flags.writeable
+
+        for arr in [arr_writeable, arr_readonly]:
+            view = arr[...]
+            view.flags.writeable = False  # make sure it is readonly
+            arr.flags.writeable = False
+            assert not arr.flags.writeable
+
+            with assert_raises(ValueError):
+                view.flags.writeable = True
+
+            with warnings.catch_warnings():
+                warnings.simplefilter("error", DeprecationWarning)
+                with assert_raises(DeprecationWarning):
+                    arr.flags.writeable = True
+
+            with assert_warns(DeprecationWarning):
+                arr.flags.writeable = True
+
+
     def test_otherflags(self):
         assert_equal(self.a.flags.carray, True)
         assert_equal(self.a.flags['C'], True)