bpo-32117: Allow tuple unpacking in return and yield statements (gh-4509)

Iterable unpacking is now allowed without parentheses in yield and return
statements, e.g. ``yield 1, 2, 3, *rest``. Thanks to David Cuthbert for the
change and jChapman for added tests.

Grammar/Grammar

@@ -50,7 +50,7 @@ pass_stmt: 'pass'
 flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt
 break_stmt: 'break'
 continue_stmt: 'continue'
-return_stmt: 'return' [testlist]
+return_stmt: 'return' [testlist_star_expr]
 yield_stmt: yield_expr
 raise_stmt: 'raise' [test ['from' test]]
 import_stmt: import_name | import_from
@@ -147,4 +147,4 @@ comp_if: 'if' test_nocond [comp_iter]
 encoding_decl: NAME
 
 yield_expr: 'yield' [yield_arg]
-yield_arg: 'from' test | testlist
+yield_arg: 'from' test | testlist_star_expr

Lib/test/test_grammar.py

@@ -824,11 +824,17 @@ class GrammarTests(unittest.TestCase):
         test_inner()
 
     def test_return(self):
-        # 'return' [testlist]
+        # 'return' [testlist_star_expr]
         def g1(): return
         def g2(): return 1
+        def g3():
+            z = [2, 3]
+            return 1, *z
+
         g1()
         x = g2()
+        y = g3()
+        self.assertEqual(y, (1, 2, 3), "unparenthesized star expr return")
         check_syntax_error(self, "class foo:return 1")
 
     def test_break_in_finally(self):
@@ -981,6 +987,9 @@ class GrammarTests(unittest.TestCase):
         def g(): f((yield 1), 1)
         def g(): f((yield from ()))
         def g(): f((yield from ()), 1)
+        # Do not require parenthesis for tuple unpacking
+        def g(): rest = 4, 5, 6; yield 1, 2, 3, *rest
+        self.assertEquals(list(g()), [(1, 2, 3, 4, 5, 6)])
         check_syntax_error(self, "def g(): f(yield 1)")
         check_syntax_error(self, "def g(): f(yield 1, 1)")
         check_syntax_error(self, "def g(): f(yield from ())")

Misc/NEWS.d/next/Core and Builtins/2017-11-22-15-43-14.bpo-32117.-vloh8.rst

+Iterable unpacking is now allowed without parenthesis in yield and return
+statements, e.g. ``yield 1, 2, 3, *rest``. Thanks to David Cuthbert for the
+change and jChapman for added tests.

Python/graminit.c

@@ -613,7 +613,7 @@ static arc arcs_25_0[1] = {
     {75, 1},
 };
 static arc arcs_25_1[2] = {
-    {9, 2},
+    {47, 2},
     {0, 1},
 };
 static arc arcs_25_2[1] = {
@@ -1900,7 +1900,7 @@ static state states_85[3] = {
 };
 static arc arcs_86_0[2] = {
     {77, 1},
-    {9, 2},
+    {47, 2},
 };
 static arc arcs_86_1[1] = {
     {26, 2},
@@ -2087,7 +2087,7 @@ static dfa dfas[87] = {
     {341, "yield_expr", 0, 3, states_85,
      "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\200\000"},
     {342, "yield_arg", 0, 3, states_86,
-     "\000\040\200\000\000\000\000\000\000\040\010\000\000\000\020\002\000\300\220\050\037\000\000"},
+     "\000\040\200\000\002\000\000\000\000\040\010\000\000\000\020\002\000\300\220\050\037\000\000"},
 };
 static label labels[177] = {
     {0, "EMPTY"},