Very subtle syntax change: in a list comprehension, the testlist in

"for <var> in <testlist> may no longer be a single test followed by
a comma.  This solves SF bug #431886.  Note that if the testlist
contains more than one test, a trailing comma is still allowed, for
maximum backward compatibility; but this example is not:

    [(x, y) for x in range(10), for y in range(10)]
                              ^

The fix involved creating a new nonterminal 'testlist_safe' whose
definition doesn't allow the trailing comma if there's only one test:

    testlist_safe: test [(',' test)+ [',']]

Grammar/Grammar

@@ -89,6 +89,7 @@ subscript: '.' '.' '.' | test | [test] ':' [test] [sliceop]
 sliceop: ':' [test]
 exprlist: expr (',' expr)* [',']
 testlist: test (',' test)* [',']
+testlist_safe: test [(',' test)+ [',']]
 dictmaker: test ':' test (',' test ':' test)* [',']
 
 classdef: 'class' NAME ['(' testlist ')'] ':' suite
@@ -97,5 +98,5 @@ arglist: (argument ',')* (argument [',']| '*' test [',' '**' test] | '**' test)
 argument: [test '='] test	# Really [keyword '='] test
 
 list_iter: list_for | list_if
-list_for: 'for' exprlist 'in' testlist [list_iter]
+list_for: 'for' exprlist 'in' testlist_safe [list_iter]
 list_if: 'if' test [list_iter]

Include/graminit.h

@@ -56,10 +56,11 @@
 #define sliceop 311
 #define exprlist 312
 #define testlist 313
-#define dictmaker 314
-#define classdef 315
-#define arglist 316
-#define argument 317
-#define list_iter 318
-#define list_for 319
-#define list_if 320
+#define testlist_safe 314
+#define dictmaker 315
+#define classdef 316
+#define arglist 317
+#define argument 318
+#define list_iter 319
+#define list_for 320
+#define list_if 321

Modules/parsermodule.c

@@ -1039,6 +1039,14 @@ validate_testlist(node *tree)
 }
 
 
+static int
+validate_testlist_safe(node *tree)
+{
+    return (validate_repeating_list(tree, testlist_safe,
+                                    validate_test, "testlist_safe"));
+}
+
+
 /* '*' NAME [',' '**' NAME] | '**' NAME
  */
 static int
@@ -1218,7 +1226,7 @@ validate_list_for(node *tree)
         res = (validate_name(CHILD(tree, 0), "for")
                && validate_exprlist(CHILD(tree, 1))
                && validate_name(CHILD(tree, 2), "in")
-               && validate_testlist(CHILD(tree, 3)));
+               && validate_testlist_safe(CHILD(tree, 3)));
 
     return res;
 }

Python/compile.c

@@ -3710,6 +3710,7 @@ com_node(struct compiling *c, node *n)
 	/* Expression nodes */
 
 	case testlist:
+	case testlist_safe:
 		com_list(c, n, 0);
 		break;
 	case test: