bpo-42316: Allow unparenthesized walrus operator in indexes (GH-23317)

Grammar/python.gram

@@ -491,7 +491,7 @@ slices[expr_ty]:
     | a[asdl_expr_seq*]=','.slice+ [','] { _Py_Tuple(a, Load, EXTRA) }
 slice[expr_ty]:
     | a=[expression] ':' b=[expression] c=[':' d=[expression] { d }] { _Py_Slice(a, b, c, EXTRA) }
-    | a=expression { a }
+    | a=named_expression { a }
 atom[expr_ty]:
     | NAME
     | 'True' { _Py_Constant(Py_True, NULL, EXTRA) }

Lib/test/test_named_expressions.py

@@ -271,6 +271,27 @@ class NamedExpressionAssignmentTest(unittest.TestCase):
         fib = {(c := a): (a := b) + (b := a + c) - b for __ in range(6)}
         self.assertEqual(fib, {1: 2, 2: 3, 3: 5, 5: 8, 8: 13, 13: 21})
 
+    def test_named_expression_assignment_17(self):
+        a = [1]
+        element = a[b:=0]
+        self.assertEqual(b, 0)
+        self.assertEqual(element, a[0])
+
+    def test_named_expression_assignment_18(self):
+        class TwoDimensionalList:
+            def __init__(self, two_dimensional_list):
+                self.two_dimensional_list = two_dimensional_list
+
+            def __getitem__(self, index):
+                return self.two_dimensional_list[index[0]][index[1]]
+
+        a = TwoDimensionalList([[1], [2]])
+        element = a[b:=0, c:=0]
+        self.assertEqual(b, 0)
+        self.assertEqual(c, 0)
+        self.assertEqual(element, a.two_dimensional_list[b][c])
+
+
 
 class NamedExpressionScopeTest(unittest.TestCase):
 

Misc/NEWS.d/next/Core and Builtins/2020-11-16-17-30-03.bpo-42316._DdmpQ.rst

@@ -0,0 +1 @@
+Allow an unparenthesized walrus in subscript indexes.

Parser/parser.c

@@ -10639,7 +10639,7 @@ slices_rule(Parser *p)
     return _res;
 }
 
-// slice: expression? ':' expression? [':' expression?] | expression
+// slice: expression? ':' expression? [':' expression?] | named_expression
 static expr_ty
 slice_rule(Parser *p)
 {
@@ -10701,18 +10701,18 @@ slice_rule(Parser *p)
         D(fprintf(stderr, "%*c%s slice[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "expression? ':' expression? [':' expression?]"));
     }
-    { // expression
+    { // named_expression
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> slice[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "expression"));
+        D(fprintf(stderr, "%*c> slice[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "named_expression"));
         expr_ty a;
         if (
-            (a = expression_rule(p))  // expression
+            (a = named_expression_rule(p))  // named_expression
         )
         {
-            D(fprintf(stderr, "%*c+ slice[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "expression"));
+            D(fprintf(stderr, "%*c+ slice[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "named_expression"));
             _res = a;
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -10723,7 +10723,7 @@ slice_rule(Parser *p)
         }
         p->mark = _mark;
         D(fprintf(stderr, "%*c%s slice[%d-%d]: %s failed!\n", p->level, ' ',
-                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "expression"));
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "named_expression"));
     }
     _res = NULL;
   done: