gh-123440: Improve error message for `except as` used with not a name (#123442)

Grammar/python.gram

@@ -434,7 +434,7 @@ except_star_block[excepthandler_ty]:
     | invalid_except_star_stmt_indent
     | 'except' '*' e=expression t=['as' z=NAME { z }] ':' b=block {
         _PyAST_ExceptHandler(e, (t) ? ((expr_ty) t)->v.Name.id : NULL, b, EXTRA) }
-    | invalid_except_stmt
+    | invalid_except_star_stmt
 finally_block[asdl_stmt_seq*]:
     | invalid_finally_stmt
     | 'finally' &&':' a=block { a }
@@ -1340,11 +1340,21 @@ invalid_try_stmt:
     | 'try' ':' block* except_star_block+ a='except' [expression ['as' NAME]] ':' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot have both 'except' and 'except*' on the same 'try'") }
 invalid_except_stmt:
-    | 'except' '*'? a=expression ',' expressions ['as' NAME ] ':' {
+    | 'except' a=expression ',' expressions ['as' NAME ] ':' {
         RAISE_SYNTAX_ERROR_STARTING_FROM(a, "multiple exception types must be parenthesized") }
-    | a='except' '*'? expression ['as' NAME ] NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
+    | a='except' expression ['as' NAME ] NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='except' NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
+    | 'except' expression 'as' a=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot use except statement with %s", _PyPegen_get_expr_name(a)) }
+invalid_except_star_stmt:
+    | 'except' '*' a=expression ',' expressions ['as' NAME ] ':' {
+        RAISE_SYNTAX_ERROR_STARTING_FROM(a, "multiple exception types must be parenthesized") }
+    | a='except' '*' expression ['as' NAME ] NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='except' '*' (NEWLINE | ':') { RAISE_SYNTAX_ERROR("expected one or more exception types") }
+    | 'except' '*' expression 'as' a=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot use except* statement with %s", _PyPegen_get_expr_name(a)) }
 invalid_finally_stmt:
     | a='finally' ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'finally' statement on line %d", a->lineno) }

Lib/test/test_syntax.py

@@ -1359,6 +1359,36 @@ Custom error message for try block mixing except and except*
    Traceback (most recent call last):
    SyntaxError: cannot have both 'except' and 'except*' on the same 'try'
 
+Better error message for using `except as` with not a name:
+
+   >>> try:
+   ...    pass
+   ... except TypeError as obj.attr:
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: cannot use except statement with attribute
+
+   >>> try:
+   ...    pass
+   ... except TypeError as obj[1]:
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: cannot use except statement with subscript
+
+   >>> try:
+   ...    pass
+   ... except* TypeError as (obj, name):
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: cannot use except* statement with tuple
+
+   >>> try:
+   ...    pass
+   ... except* TypeError as 1:
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: cannot use except* statement with literal
+
 Ensure that early = are not matched by the parser as invalid comparisons
    >>> f(2, 4, x=34); 1 $ 2
    Traceback (most recent call last):
@@ -2770,6 +2800,23 @@ while 1:
         with self.assertRaises(SyntaxError):
             compile(source, "<string>", "exec")
 
+    def test_except_stmt_invalid_as_expr(self):
+        self._check_error(
+            textwrap.dedent(
+                """
+                try:
+                    pass
+                except ValueError as obj.attr:
+                    pass
+                """
+            ),
+            errtext="cannot use except statement with attribute",
+            lineno=4,
+            end_lineno=4,
+            offset=22,
+            end_offset=22 + len("obj.attr"),
+        )
+
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

Misc/NEWS.d/next/Core_and_Builtins/2024-08-28-22-42-51.gh-issue-123440.yOFB0N.rst

@@ -0,0 +1 @@
+Improve :exc:`SyntaxError` message for using ``except as`` with not a name.