PEP 758 – Allow except
and except*
expressions without parentheses
- Author:
- Pablo Galindo <pablogsal at python.org>, Brett Cannon <brett at python.org>
- PEP-Delegate:
- TBD
- Status:
- Draft
- Type:
- Standards Track
- Created:
- 30-Sep-2024
- Python-Version:
- 3.14
- Post-History:
- 02-Oct-2024
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:
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:
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:
try:
...
except* ExceptionA, ExceptionB, ExceptionC:
...
Both forms will also allow the use of the as
clause to capture the exception
instance as before:
try:
...
except ExceptionA, ExceptionB, ExceptionC as e:
...
Rationale
The decision to allow unparenthesized except
blocks is based on the
following considerations:
- Simplicity: Removing the requirement for parentheses simplifies the syntax, making it more consistent with other parts of the language.
- Readability: In cases where many exceptions are being caught, the removal of parentheses can improve readability by reducing visual clutter.
- 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:
except_block:
| 'except' expressions ['as' NAME] ':' block
| 'except' ':' block
except_star_block
| 'except' '*' expressions ['as' NAME] ':' block
This allows both the current parenthesized syntax and the new unparenthesized syntax:
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:
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:
# 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
- Allowing mixed parenthesized and unparenthesized syntax:
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
Copyright
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Source: https://github.com/python/peps/blob/main/peps/pep-0758.rst
Last modified: 2024-10-02 23:11:48 GMT