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

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())