bpo-42374: Allow unparenthesized walrus in genexps

Grammar/python.gram

@@ -517,7 +517,7 @@ group[expr_ty]:
     | '(' a=(yield_expr | named_expression) ')' { a }
     | invalid_group
 genexp[expr_ty]:
-    | '(' a=expression ~ b=for_if_clauses ')' { _Py_GeneratorExp(a, b, EXTRA) }
+    | '(' a=named_expression ~ b=for_if_clauses ')' { _Py_GeneratorExp(a, b, EXTRA) }
     | invalid_comprehension
 set[expr_ty]: '{' a=expressions_list '}' { _Py_Set(a, EXTRA) }
 setcomp[expr_ty]:

Lib/test/test_named_expressions.py

@@ -513,6 +513,15 @@ def g():
             self.assertEqual(nonlocal_var, None)
         f()
 
+    def test_named_expression_scope_in_genexp(self):
+        a = 1
+        b = [1, 2, 3, 4]
+        genexp = (c := i + a for i in b)
+
+        self.assertNotIn("c", locals())
+        for idx, elem in enumerate(genexp):
+            self.assertEqual(elem, b[idx] + a)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2020-11-16-17-57-09.bpo-42374.t7np1E.rst

@@ -0,0 +1,2 @@
+Fix a regression introduced by the new parser, where an unparenthesized walrus operator
+was not allowed within generator expressions.

Parser/parser.c

@@ -11357,7 +11357,7 @@ group_rule(Parser *p)
     return _res;
 }
 
-// genexp: '(' expression ~ for_if_clauses ')' | invalid_comprehension
+// genexp: '(' named_expression ~ for_if_clauses ')' | invalid_comprehension
 static expr_ty
 genexp_rule(Parser *p)
 {
@@ -11377,12 +11377,12 @@ genexp_rule(Parser *p)
     UNUSED(_start_lineno); // Only used by EXTRA macro
     int _start_col_offset = p->tokens[_mark]->col_offset;
     UNUSED(_start_col_offset); // Only used by EXTRA macro
-    { // '(' expression ~ for_if_clauses ')'
+    { // '(' named_expression ~ for_if_clauses ')'
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> genexp[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "'(' expression ~ for_if_clauses ')'"));
+        D(fprintf(stderr, "%*c> genexp[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "'(' named_expression ~ for_if_clauses ')'"));
         int _cut_var = 0;
         Token * _literal;
         Token * _literal_1;
@@ -11391,7 +11391,7 @@ genexp_rule(Parser *p)
         if (
             (_literal = _PyPegen_expect_token(p, 7))  // token='('
             &&
-            (a = expression_rule(p))  // expression
+            (a = named_expression_rule(p))  // named_expression
             &&
             (_cut_var = 1)
             &&
@@ -11400,7 +11400,7 @@ genexp_rule(Parser *p)
             (_literal_1 = _PyPegen_expect_token(p, 8))  // token=')'
         )
         {
-            D(fprintf(stderr, "%*c+ genexp[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'(' expression ~ for_if_clauses ')'"));
+            D(fprintf(stderr, "%*c+ genexp[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'(' named_expression ~ for_if_clauses ')'"));
             Token *_token = _PyPegen_get_last_nonnwhitespace_token(p);
             if (_token == NULL) {
                 D(p->level--);
@@ -11420,7 +11420,7 @@ genexp_rule(Parser *p)
         }
         p->mark = _mark;
         D(fprintf(stderr, "%*c%s genexp[%d-%d]: %s failed!\n", p->level, ' ',
-                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'(' expression ~ for_if_clauses ')'"));
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'(' named_expression ~ for_if_clauses ')'"));
         if (_cut_var) {
             D(p->level--);
             return NULL;