PEP 758: Allow except and except* expressions without parenth…

…eses (#4008)

.github/CODEOWNERS

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+PEP: 758
+Title: Allow ``except`` and ``except*`` expressions without parentheses
+Author: Pablo Galindo <[email protected]>, Brett Cannon <[email protected]>
+PEP-Delegate: TBD
+Status: Draft
+Type: Standards Track
+Created: 30-Sep-2024
+Python-Version: 3.14
+
+
+Abstract
+========
+
+This PEP [1]_ proposes to allow unparenthesized ``except`` and ``except*``
+blocks in Python's exception handling syntax. Currently, when catching multiple
+exceptions, parentheses are required around the exception types. This was a
+Python 2 remnant. This PEP suggests allowing the omission of these parentheses,
+simplifying the syntax, making it more consistent with other parts of the syntax
+that make parentheses optional, and improving readability in certain cases.
+
+
+Motivation
+==========
+
+The current syntax for catching multiple exceptions requires parentheses in the
+``except`` expression (equivalently for the ``except*`` expression). For
+example:
+
+.. code-block:: python
+
+    try:
+        ...
+    except (ExceptionA, ExceptionB, ExceptionC):
+        ...
+
+While this syntax is clear and unambiguous, it can be seen as unnecessarily
+verbose in some cases, especially when catching a large number of exceptions. By
+allowing the omission of parentheses, we can simplify the syntax:
+
+.. code-block:: python
+
+    try:
+        ...
+    except ExceptionA, ExceptionB, ExceptionC:
+        ...
+
+This change would bring the syntax more in line with other comma-separated lists
+in Python, such as function arguments, generator expressions inside of a
+function call, and tuple literals, where parentheses are optional.
+
+The same change would apply to ``except*`` expressions. For example:
+
+.. code-block:: python
+
+    try:
+        ...
+    except* ExceptionA, ExceptionB, ExceptionC:
+        ...
+
+Both forms will also allow the use of the ``as`` clause to capture the exception
+instance as before:
+
+.. code-block:: python
+
+    try:
+        ...
+    except ExceptionA, ExceptionB, ExceptionC as e:
+        ...
+
+
+Rationale
+=========
+
+The decision to allow unparenthesized ``except`` blocks is based on the
+following considerations:
+
+1. Simplicity: Removing the requirement for parentheses simplifies the syntax,
+making it more consistent with other parts of the language.
+
+2. Readability: In cases where many exceptions are being caught, the removal of
+parentheses can improve readability by reducing visual clutter.
+
+3. Consistency: This change makes the ``except`` clause more consistent with other parts of Python where unambiguous, comma-separated lists don't require parentheses.
+
+Specification
+=============
+
+The syntax for the except clause will be modified to allow an unparenthesized
+list of exception types. The grammar will be updated as follows:
+
+.. code-block:: peg
+
+    except_block[excepthandler_ty]:
+        | invalid_except_stmt_indent
+        | 'except' e=expressions t=['as' z=NAME { z }] ':' b=block {
+            _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
+    except_star_block[excepthandler_ty]:
+        | invalid_except_star_stmt_indent
+        | 'except' '*' e=expressions t=['as' z=NAME { z }] ':' b=block {
+            _PyAST_ExceptHandler(e, (t) ? ((expr_ty) t)->v.Name.id : NULL, b, EXTRA) }
+        | invalid_except_star_stmt
+
+
+This allows both the current parenthesized syntax and the new unparenthesized
+syntax:
+
+.. code-block:: python
+
+    try:
+        ...
+    except (ExceptionA, ExceptionB):  # Still valid
+        ...
+    except ExceptionC, ExceptionD:    # New syntax
+        ...
+
+The semantics of exception handling remain unchanged. The interpreter will catch
+any of the listed exceptions, regardless of whether they are parenthesized or
+not.
+
+
+Backwards Compatibility
+=======================
+
+This change is fully backwards compatible. All existing code using parenthesized
+``except`` and ``except*`` blocks will continue to work without modification.
+The new syntax is purely additive and does not break any existing code.
+
+It's worth noting that in Python 2 the unparenthesized syntax was allowed with
+two elements, but had different semantics, in which the first element of the
+list was used as the exception type and the second element as the capture
+variable. This change does not reintroduce the Python 2 semantics, and the
+unparenthesized syntax will behave identically to the parenthesized version.
+
+
+Security Implications
+=====================
+
+There are no known security implications for this change. The semantics of
+exception handling remain the same, and this is purely a syntactic change.
+
+
+How to Teach This
+=================
+
+For new Python users, the unparenthesized syntax can be taught as the standard
+way to catch multiple exceptions:
+
+.. code-block:: python
+
+    try:
+        risky_operation()
+    except ValueError, TypeError, OSError:
+        handle_errors()
+
+For experienced users, it can be introduced as a new, optional syntax that can
+be used interchangeably with the parenthesized version. Documentation should
+note that both forms are equivalent:
+
+.. code-block:: python
+
+    # These are equivalent:
+    except (ValueError, TypeError):
+        ...
+
+    except ValueError, TypeError:
+        ...
+
+It should be emphasized that this is purely a syntactic change and does not
+affect the behaviour of exception handling.
+
+
+Reference Implementation
+========================
+
+A proof-of-concept implementation is available at
+https://github.com/pablogsal/cpython/commits/notuples/. This implementation
+modifies the Python parser to accept the new syntax and ensures that it behaves
+identically to the parenthesized version.
+
+
+Rejected Ideas
+==============
+
+1. Allowing mixed parenthesized and unparenthesized syntax:
+
+   .. code-block:: python
+
+      try:
+          ...
+      except (ValueError, TypeError), OSError:
+         ...
+
+   This was rejected due to the potential for confusion and to maintain a clear
+   distinction between the two styles.
+
+Footnotes
+=========
+
+.. [1] Originally named "Parenthetically Speaking, We Don't Need 'Em"
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.