gh-133192: Fix regression with PEP 758 parsing on older feature_version

Author: sobolevn

Grammar/python.gram

@@ -443,26 +443,30 @@ try_stmt[stmt_ty]:
 
 except_block[excepthandler_ty]:
     | invalid_except_stmt_indent
+    | 'except' e=expression ':' b=block {
+        _PyAST_ExceptHandler(e, NULL, b, EXTRA) }
+    | 'except' e=expression 'as' t=NAME ':' b=block {
+        _PyAST_ExceptHandler(e, ((expr_ty) t)->v.Name.id, b, EXTRA) }
     | 'except' e=expressions ':' b=block {
         CHECK_VERSION(
             excepthandler_ty, 
             14, 
-            "except expressions without parentheses", 
-            _PyAST_ExceptHandler(e,  NULL, b, EXTRA)) }
-    | 'except' e=expression 'as' t=NAME  ':' b=block {
-        _PyAST_ExceptHandler(e, ((expr_ty) t)->v.Name.id, b, EXTRA) }
+            "except expressions without parentheses are", 
+            _PyAST_ExceptHandler(e, NULL, b, EXTRA)) }
     | 'except' ':' b=block { _PyAST_ExceptHandler(NULL, NULL, b, EXTRA) }
     | invalid_except_stmt
 except_star_block[excepthandler_ty]:
     | invalid_except_star_stmt_indent
+    | 'except' '*' e=expression ':' b=block {
+        _PyAST_ExceptHandler(e, NULL, b, EXTRA) }
+    | 'except' '*' e=expression 'as' t=NAME ':' b=block {
+        _PyAST_ExceptHandler(e, ((expr_ty) t)->v.Name.id, b, EXTRA) }
     | 'except' '*' e=expressions ':' b=block {
         CHECK_VERSION(
             excepthandler_ty, 
             14, 
-            "except expressions without parentheses", 
+            "except expressions without parentheses are", 
             _PyAST_ExceptHandler(e,  NULL, b, EXTRA)) }
-    | 'except' '*' e=expression 'as' t=NAME ':' b=block {
-        _PyAST_ExceptHandler(e, ((expr_ty) t)->v.Name.id, b, EXTRA) }
     | invalid_except_star_stmt
 finally_block[asdl_stmt_seq*]:
     | invalid_finally_stmt

Lib/test/test_ast/test_ast.py

@@ -692,6 +692,63 @@ def test_pep758_except_without_parens(self):
         with self.assertRaises(SyntaxError):
             ast.parse(code, feature_version=(3, 13))
 
+    def test_pep758_except_with_single_expr(self):
+        single_expr = textwrap.dedent("""
+            try:
+                ...
+            except{0} TypeError:
+                ...
+        """)
+
+        single_expr_with_as = textwrap.dedent("""
+            try:
+                ...
+            except{0} TypeError as exc:
+                ...
+        """)
+
+        single_tuple_expr = textwrap.dedent("""
+            try:
+                ...
+            except{0} (TypeError,):
+                ...
+        """)
+
+        single_tuple_expr_with_as = textwrap.dedent("""
+            try:
+                ...
+            except{0} (TypeError,) as exc:
+                ...
+        """)
+
+        single_parens_expr = textwrap.dedent("""
+            try:
+                ...
+            except{0} (TypeError):
+                ...
+        """)
+
+        single_parens_expr_with_as = textwrap.dedent("""
+            try:
+                ...
+            except{0} (TypeError) as exc:
+                ...
+        """)
+
+        for code in [
+            single_expr,
+            single_expr_with_as,
+            single_tuple_expr,
+            single_tuple_expr_with_as,
+            single_parens_expr,
+            single_parens_expr_with_as,
+        ]:
+            for star in [True, False]:
+                code = code.format('*' if star else '')
+                with self.subTest(code=code, star=star):
+                    ast.parse(code, feature_version=(3, 14))
+                    ast.parse(code, feature_version=(3, 13))
+
     def test_pep758_except_star_without_parens(self):
         code = textwrap.dedent("""
             try: