pythongh-123440: Improve error message for except as used with not …

…a name

Grammar/python.gram

@@ -430,11 +430,13 @@ except_block[excepthandler_ty]:
  _PyAST_ExceptHandler(e, (t) ? ((expr_ty) t)->v.Name.id : NULL, b, EXTRA) }
  | 'except' ':' b=block { _PyAST_ExceptHandler(NULL, NULL, b, EXTRA) }
  | invalid_except_stmt
+ | invalid_except_as_expr
 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_as_expr
 finally_block[asdl_stmt_seq*]:
  | invalid_finally_stmt
  | 'finally' &&':' a=block { a }
@@ -1345,6 +1347,14 @@ invalid_except_stmt:
  | a='except' '*'? expression ['as' NAME ] NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
  | a='except' NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
  | a='except' '*' (NEWLINE | ':') { RAISE_SYNTAX_ERROR("expected one or more exception types") }
+invalid_except_as_expr:
+ | '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_as_expr:
+ | '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 @@
  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):

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.