python-peps/peps/pep-0758.rst

PEP: 758
Title: Allow ``except`` and ``except*`` expressions without parentheses
Author: Pablo Galindo <pablogsal@python.org>, Brett Cannon <brett@python.org>
PEP-Delegate: TBD
Status: Draft
Type: Standards Track
Created: 30-Sep-2024
Python-Version: 3.14
Post-History: `02-Oct-2024 <https://discuss.python.org/t/66453>`__

Abstract
========

This PEP [1]_ proposes to allow unparenthesized ``except`` and ``except*``
blocks in Python's exception handling syntax only when not using the ``as``
clause. 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:
        ...

When using the ``as`` clause to capture the exception instance paretheses must
be used as before. Some users have expressed that they would find it confusing not
to require parentheses as it would be unclear what exactly is being assigned to
the target since in other parts of the language multiple ``as`` clauses can be used
in similar situations (like in imports and context managers). This means that if
an ``as`` clause its being added to the previous example it must be done as:

.. 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:
        | 'except' expressions ':' block
        | 'except' expression 'as' NAME ':' block
        | 'except' ':' block

    except_star_block
        | 'except' '*' expressions ':' block 
        | 'except' '*' expression 'as' NAME ':' block 

This allows both the current parenthesized syntax and the new unparenthesized
syntax while requiring parentheses when the ``as`` keyword is used:

.. code-block:: python

    try:
        ...
    except (ExceptionA, ExceptionB):  # Still valid
        ...
    except ExceptionC, ExceptionD:    # New syntax
        ...
    except (ExceptionE, ExceptionF) as e: # Parentheses still required
        ...

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.

Deferred Ideas
==============

1. Allowing unparenthesized expressions when the ``as`` keyword is used. The reason
   we have decided to defer this particular form given that there isn't clear
   consensus either way and there are reasonable arguments for both positions, the safest
   approach is keeping the parentheses requirement since it can be removed later if users
   find the disconnect too acute, while taking it out and users deciding it was a bad idea
   doesn’t make it easy to put back.

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.