Python: fix PEP 758 except A, B: in the default parser

The grammar rule shared by both readings is

    except_clause: 'except' [test [(',' | 'as') test]]

and `visit_except_clause` ignored the separator token, always treating the
fourth child as an alias to bind. So `except A, B:` extracted `B` as a Store
rather than a use, which is the Python 2 reading. Queries that reason about
whether a name is used then report false positives; `py/unused-import` flags
the import of `B` as unused.

The tree-sitter parser already extracts this as a tuple of exception types
(#20990), so the two parsers disagreed. `tests/parser/exceptions_relaxed.py`
is an unsuffixed parser test, which asserts the two parsers produce identical
ASTs; it fails without this change.

With the fix, the default parser reproduces the existing
`tests/parser/exceptions_new.expected` byte for byte, and of the 37 parser
test files only the two containing PEP 758 syntax change at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: aausch

python/extractor/semmle/python/parser/ast.py

@@ -981,7 +981,16 @@ def visit_except_clause(self, node):
         if len(node.children) > 1:
             type = self.visit(node.children[1], LOAD)
         if len(node.children) > 3:
-            name = self.visit(node.children[3], STORE)
+            if is_token(node.children[2], "as"):
+                name = self.visit(node.children[3], STORE)
+            else:
+                # PEP 758 (Python 3.14+): `except A, B:` is an unparenthesized
+                # tuple of exception types, not a Python 2 alias binding. The
+                # grammar rule `'except' [test [(',' | 'as') test]]` is shared
+                # between both readings, so the separator token decides.
+                elts = [type, self.visit(node.children[3], LOAD)]
+                type = ast.Tuple(elts, LOAD)
+                set_location(type, node.children[1].start, node.children[3].end)
         return type, name
 
     def visit_del_stmt(self, node):

python/extractor/tests/parser/exceptions_relaxed.py

@@ -0,0 +1,10 @@
+try:
+    a
+except b, c:
+    d
+except (e, f):
+    g
+except h as i:
+    j
+except k:
+    l

python/ql/lib/change-notes/2026-08-19-legacy-parser-relaxed-except.md

@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* Fixed the extraction of PEP 758 `except A, B:` clauses by the default (non-tree-sitter) Python parser. Previously the second exception type was extracted as a Python 2 style alias binding, so it was recorded as a `Store` rather than a use. This caused false positives from queries that reason about whether a name is used, such as `py/unused-import`.