Python: Add support for PEP-758 exception syntax #20990

tausbn · 2025-12-08T17:11:12Z

~~This PR is stacked on top of #20708, so that one should be reviewed first.~~ That PR has now been merged.

See https://peps.python.org/pep-0758/ for more details.

We implement this by extending the syntax for exceptions and exception groups so that the type field can now contain either an expression (which matches the old behaviour), or a comma-separated list of at least two elements (representing the new behaviour).

We model the latter case using a new node type exception_list, which in tsg-python is simply mapped to a tuple. This means it matches the existing behaviour (when the tuple is surrounded by parentheses) exactly, hence we don't need to change any other code.

As a consequence of this, however, we cannot directly parse the Python 2.7 syntax except Foo, e: ... as except Foo as e: ..., as this would introduce an ambiguity in the grammar. Thus, we have removed support for the (deprecated) 2.7-style syntax, and only allow as to indicate binding of the exception. The syntax except Foo, e: ... continues to be parsed (in particular, it's not suddenly a syntax error), but it will be parsed as if it were except (Foo, e): ..., which may not give the correct results.

In principle we could extend the QL libraries to account for this case (specifically when analysing Python 2 code). In practice, however, I expect this to have a minor impact on results, and not worth the additional investment at this time.

See https://peps.python.org/pep-0758/ for more details. We implement this by extending the syntax for exceptions and exception groups so that the `type` field can now contain either an expression (which matches the old behaviour), or a comma-separated list of at least two elements (representing the new behaviour). We model the latter case using a new node type `exception_list`, which in `tsg-python` is simply mapped to a tuple. This means it matches the existing behaviour (when the tuple is surrounded by parentheses) exactly, hence we don't need to change any other code. As a consequence of this, however, we cannot directly parse the Python 2.7 syntax `except Foo, e: ...` as `except Foo as e: ...`, as this would introduce an ambiguity in the grammar. Thus, we have removed support for the (deprecated) 2.7-style syntax, and only allow `as` to indicate binding of the exception. The syntax `except Foo, e: ...` continues to be parsed (in particular, it's not suddenly a syntax error), but it will be parsed as if it were `except (Foo, e): ...`, which may not give the correct results. In principle we could extend the QL libraries to account for this case (specifically when analysing Python 2 code). In practice, however, I expect this to have a minor impact on results, and not worth the additional investment at this time.

Note in particular that the `exceptions.py` test is unaffected.

Copilot

Pull request overview

This PR adds support for PEP-758's relaxed exception syntax, allowing exception types to be specified as comma-separated lists without requiring parentheses (e.g., except A, B, C: instead of except (A, B, C):).

Key Changes:

Introduced a new exception_list grammar node type that accepts comma-separated expressions
Updated except_clause and except_group_clause to accept either an expression or an exception_list for the type field
Removed support for Python 2.7's deprecated except Foo, e: syntax for exception binding (now only as is supported for binding)

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
`python/ql/lib/change-notes/2025-12-09-python-support-relaxed-except-syntax.md`	Documents the new PEP-758 support as a feature addition
`python/extractor/tsg-python/tsp/src/node-types.json`	Defines the new `exception_list` node type and updates exception clause type fields to accept it
`python/extractor/tsg-python/tsp/src/grammar.json`	Generated grammar definition matching the changes in grammar.js
`python/extractor/tsg-python/tsp/grammar.js`	Implements the core grammar changes: new `exception_list` rule and updated exception clause rules
`python/extractor/tsg-python/python.tsg`	Maps `exception_list` nodes to Tuple AST nodes and handles element relationships
`python/extractor/tests/parser/exceptions_new.py`	Test cases covering the new relaxed syntax alongside old parenthesized syntax
`python/extractor/tests/parser/exceptions_new.expected`	Expected AST output for the new exception syntax tests
`python/extractor/tests/parser/exception_groups_new.py`	Adds test case for exception groups with the new syntax (`except* x, y:`)
`python/extractor/tests/parser/exception_groups_new.expected`	Expected AST output including the new exception group syntax test

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Copilot · 2026-01-06T13:46:30Z

python/extractor/tests/parser/exception_groups_new.py

+
+try:
+    pass
+except* x, y:


Syntax Error (in Python 3).

Suggested change

except* x, y:

except* (x, y):

See https://peps.python.org/pep-0758/ .

Copilot · 2026-01-06T13:46:30Z

python/extractor/tests/parser/exceptions_new.py

@@ -0,0 +1,8 @@
+try:
+    pass
+except a, b: # new, relaxed syntax


This statement is unreachable.

Suggested change

except a, b: # new, relaxed syntax

except a as b: # new, relaxed syntax

geoffw0 · 2026-01-07T10:05:53Z

In principle we could extend the QL libraries to account for this case (specifically when analysing Python 2 code).

What proportion of users are still using Python 2 these days?

geoffw0

Looks plausible and has tests. The DCA run failed.

geoffw0 · 2026-01-07T10:11:11Z

python/extractor/tsg-python/tsp/src/grammar.json

+          }
+        }
+      ]
+    },


If this file is generated, it should be possible to mark it as such, then it will be hidden by default in these diffs.

yoff

Looks generally reasonable. I guess we do have a breaking change, but I am not sure we have promised for the 2.7 semantics to keep working?

yoff · 2026-01-07T11:52:42Z

python/ql/lib/change-notes/2025-12-09-python-support-relaxed-except-syntax.md

+---
+category: feature
+---
+* The extractor now supports the new, relaxed syntax `except A, B, C: ...` (which would previously have to be written as `except (A, B, C): ...`) as defined in [PEP-758](https://peps.python.org/pep-0758/).


Should we mention that we drop support for 2.7 syntax?

github-actions bot added documentation Python labels Dec 8, 2025

tausbn force-pushed the tausbn/python-support-relaxed-exception-groups branch from 545241a to 7f98e1e Compare December 16, 2025 22:58

tausbn added 4 commits January 6, 2026 13:40

Python: Regenerate parser files

4db60df

Python: Add parser test

2c83b29

Note in particular that the `exceptions.py` test is unaffected.

Python: Add change note

4a567ad

tausbn force-pushed the tausbn/python-support-relaxed-exception-groups branch from 7f98e1e to 4a567ad Compare January 6, 2026 13:41

tausbn marked this pull request as ready for review January 6, 2026 13:42

tausbn requested review from a team as code owners January 6, 2026 13:42

Copilot AI review requested due to automatic review settings January 6, 2026 13:42

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tausbn commented Dec 8, 2025 •

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	except a, b: # new, relaxed syntax
	except a as b: # new, relaxed syntax

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tausbn commented Dec 8, 2025 •

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