gh-131831: Implement PEP 758 – Allow except and except* expressions without parentheses

Author: pablogsal

Doc/reference/compound_stmts.rst

@@ -232,6 +232,8 @@ Additional information on exceptions can be found in section :ref:`exceptions`,
 and information on using the :keyword:`raise` statement to generate exceptions
 may be found in section :ref:`raise`.
 
+.. versionchanged:: 3.13
+   Support for optionally dropping grouping parentheses when using multiple exception types. See :pep:`758`.
 
 .. _except:
 
@@ -247,7 +249,8 @@ An expression-less :keyword:`!except` clause, if present, must be last;
 it matches any exception.
 
 For an :keyword:`!except` clause with an expression, the
-expression must evaluate to an exception type or a tuple of exception types.
+expression must evaluate to an exception type or a tuple of exception types. Parentheses
+can be dropped if multiple exception types are provided and the ``as`` clause is not used.
 The raised exception matches an :keyword:`!except` clause whose expression evaluates
 to the class or a :term:`non-virtual base class <abstract base class>` of the exception object,
 or to a tuple that contains such a class.

Doc/whatsnew/3.14.rst

@@ -90,6 +90,29 @@ If you encounter :exc:`NameError`\s or pickling errors coming out of
 New features
 ============
 
+PEP 758 – Allow except and except* expressions without parentheses
+------------------------------------------------------------------
+
+The :keyword:`except` and :keyword:`except*` expressions now allow
+parentheses to be omitted when there is only one exception type.
+For example the following expressions are now valid:
+
+  .. code-block:: python
+
+      try:
+          release_new_sleep_token_album()
+      except AlbumNotFound, SongsTooGoodToBeReleased:
+          print("Sorry, no new album this year.")
+
+      try:
+          release_new_sleep_token_album()
+      except* AlbumNotFound, SongsTooGoodToBeReleased:
+          print("Sorry, no new album this year.")
+
+Check :pep:`758` for more details.
+
+(Contributed by Pablo Galindo and Brett Cannin in :issue:`131831`.)
+
 .. _whatsnew314-pep649:
 
 PEP 649: deferred evaluation of annotations

Grammar/python.gram

@@ -435,14 +435,18 @@ try_stmt[stmt_ty]:
 
 except_block[excepthandler_ty]:
     | invalid_except_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) }
+    | 'except' e=expressions ':' 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' ':' b=block { _PyAST_ExceptHandler(NULL, NULL, b, EXTRA) }
     | invalid_except_stmt
 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) }
+    | 'except' '*' e=expressions ':' 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) }
     | invalid_except_star_stmt
 finally_block[asdl_stmt_seq*]:
     | invalid_finally_stmt
@@ -1356,15 +1360,15 @@ 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' 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 ] ':' {
+    | '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") }

Lib/test/test_syntax.py

@@ -1667,21 +1667,7 @@
    SyntaxError: invalid syntax
 
 Check that an multiple exception types with missing parentheses
-raise a custom exception
-
-   >>> try:
-   ...   pass
-   ... except A, B:
-   ...   pass
-   Traceback (most recent call last):
-   SyntaxError: multiple exception types must be parenthesized
-
-   >>> try:
-   ...   pass
-   ... except A, B, C:
-   ...   pass
-   Traceback (most recent call last):
-   SyntaxError: multiple exception types must be parenthesized
+raise a custom exception only when using 'as'
 
    >>> try:
    ...   pass
@@ -1700,20 +1686,6 @@
    SyntaxError: multiple exception types must be parenthesized
 
 
-   >>> try:
-   ...   pass
-   ... except* A, B:
-   ...   pass
-   Traceback (most recent call last):
-   SyntaxError: multiple exception types must be parenthesized
-
-   >>> try:
-   ...   pass
-   ... except* A, B, C:
-   ...   pass
-   Traceback (most recent call last):
-   SyntaxError: multiple exception types must be parenthesized
-
    >>> try:
    ...   pass
    ... except* A, B, C as blech: