diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS index 36b5fbed6d0..2c99583b4cc 100644 --- a/.github/CODEOWNERS +++ b/.github/CODEOWNERS @@ -638,6 +638,7 @@ peps/pep-0753.rst @warsaw # ... peps/pep-0756.rst @vstinner peps/pep-0757.rst @vstinner +peps/pep-0758.rst @pablogsal @brettcannon peps/pep-0789.rst @njsmith # ... peps/pep-0801.rst @warsaw diff --git a/peps/pep-0790.rst b/peps/pep-0758.rst similarity index 53% rename from peps/pep-0790.rst rename to peps/pep-0758.rst index b1ca9ed65ee..fe1aa055271 100644 --- a/peps/pep-0790.rst +++ b/peps/pep-0758.rst @@ -1,8 +1,7 @@ PEP: 758 -Title: Allow ``except`` expressions without parentheses +Title: Allow ``except`` and ``except*`` expressions without parentheses Author: Pablo Galindo , Brett Cannon PEP-Delegate: TBD -Discussions-To: xxxx Status: Draft Type: Standards Track Created: 30-Sep-2024 @@ -12,13 +11,20 @@ Python-Version: 3.14 Abstract ======== -This PEP [1]_ proposes to allow unparenthesized ``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. +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: +The current syntax for catching multiple exceptions requires parentheses in the +``except`` expression (equivalently for the ``except*`` expression). For +example: .. code-block:: python @@ -27,7 +33,9 @@ The current syntax for catching multiple exceptions requires parentheses in the 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: +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 @@ -36,24 +44,49 @@ While this syntax is clear and unambiguous, it can be seen as unnecessarily verb 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. +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: +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. +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. +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: +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 @@ -70,7 +103,8 @@ The syntax for the except clause will be modified to allow an unparenthesized li | invalid_except_star_stmt -This allows both the current parenthesized syntax and the new unparenthesized syntax: +This allows both the current parenthesized syntax and the new unparenthesized +syntax: .. code-block:: python @@ -81,25 +115,37 @@ This allows both the current parenthesized syntax and the new unparenthesized sy 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. +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`` blocks will continue to work without modification. The new syntax is purely additive and does not break any existing code. +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 eleements, 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. +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: +For new Python users, the unparenthesized syntax can be taught as the standard +way to catch multiple exceptions: .. code-block:: python @@ -108,7 +154,9 @@ For new Python users, the unparenthesized syntax can be taught as the standard w 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: +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 @@ -119,13 +167,17 @@ For experienced users, it can be introduced as a new, optional syntax that can b except ValueError, TypeError: ... -It should be emphasized that this is purely a syntactic change and does not affect the behaviour of exception handling. +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. +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 @@ -140,7 +192,8 @@ Rejected Ideas except (ValueError, TypeError), OSError: ... - This was rejected due to the potential for confusion and to maintain a clear distinction between the two styles. + This was rejected due to the potential for confusion and to maintain a clear + distinction between the two styles. Footnotes =========