bpo-37840: Fix handling of negative indices in bytearray_getitem() (GH-15250)

Lib/test/test_bytes.py

@@ -962,6 +962,15 @@ class BaseBytesTest:
         c = b.translate(None, delete=b'e')
         self.assertEqual(c, b'hllo')
 
+    def test_sq_item(self):
+        _testcapi = test.support.import_module('_testcapi')
+        obj = self.type2test((42,))
+        with self.assertRaises(IndexError):
+            _testcapi.sequence_getitem(obj, -2)
+        with self.assertRaises(IndexError):
+            _testcapi.sequence_getitem(obj, 1)
+        self.assertEqual(_testcapi.sequence_getitem(obj, 0), 42)
+
 
 class BytesTest(BaseBytesTest, unittest.TestCase):
     type2test = bytes

Misc/NEWS.d/next/Core and Builtins/2019-08-13-18-05-20.bpo-37840.elLCci.rst

+Fix handling of negative indices in :c:member:`~PySequenceMethods.sq_item`
+of :class:`bytearray`. Patch by Sergey Fedoseev.

Modules/_testcapimodule.c

@@ -5164,6 +5164,18 @@ test_write_unraisable_exc(PyObject *self, PyObject *args)
 }
 
 
+static PyObject *
+sequence_getitem(PyObject *self, PyObject *args)
+{
+    PyObject *seq;
+    Py_ssize_t i;
+    if (!PyArg_ParseTuple(args, "On", &seq, &i)) {
+        return NULL;
+    }
+    return PySequence_GetItem(seq, i);
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",       raise_memoryerror,               METH_NOARGS},
@@ -5413,6 +5425,7 @@ static PyMethodDef TestMethods[] = {
     {"negative_refcount", negative_refcount, METH_NOARGS},
 #endif
     {"write_unraisable_exc", test_write_unraisable_exc, METH_VARARGS},
+    {"sequence_getitem", sequence_getitem, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };
 

Objects/bytearrayobject.c

@@ -381,8 +381,6 @@ bytearray_irepeat(PyByteArrayObject *self, Py_ssize_t count)
 static PyObject *
 bytearray_getitem(PyByteArrayObject *self, Py_ssize_t i)
 {
-    if (i < 0)
-        i += Py_SIZE(self);
     if (i < 0 || i >= Py_SIZE(self)) {
         PyErr_SetString(PyExc_IndexError, "bytearray index out of range");
         return NULL;