Merge pull request #22386 from aausch/aausch/python-pep758-legacy-parser

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

Author: tausbn

python/extractor/semmle/populator.py

@@ -65,6 +65,11 @@ def main(sys_path = sys.path[:]):
     if options.language_version:
         last_version = options.language_version[-1]
         update_analysis_version(last_version)
+        # Worker processes are spawned rather than forked on macOS, so they do
+        # not inherit the value set above; they re-read it from the environment
+        # as this module did on import. Set it there too, or `--lang` would take
+        # effect in this process only, and on one platform only.
+        os.environ["CODEQL_EXTRACTOR_PYTHON_ANALYSIS_VERSION"] = last_version
 
     found_py2 = False
     if get_analysis_major_version() == 2 and options.extract_stdlib:

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

@@ -1,6 +1,7 @@
 from blib2to3.pgen2 import token
 from ast import literal_eval
 from semmle.python import ast
+from semmle.util import get_analysis_major_version
 from blib2to3.pgen2.parse import ParseError
 import sys
 
@@ -981,7 +982,20 @@ 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)
+            # The grammar rule `'except' [test [(',' | 'as') test]]` is shared
+            # between two incompatible readings of a fourth child, so the
+            # separator token and the analysis version together decide:
+            #   `except A as e:`  binds an alias, in every version;
+            #   `except A, e:`    binds an alias when extracting Python 2, where
+            #                     that is the canonical idiom;
+            #   `except A, B:`    is an unparenthesized tuple of exception types
+            #                     otherwise -- PEP 758, Python 3.14+.
+            if is_token(node.children[2], "as") or get_analysis_major_version() == 2:
+                name = self.visit(node.children[3], STORE)
+            else:
+                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/semmle/util.py

@@ -10,7 +10,7 @@
 
 #Semantic version of extractor.
 #Update this if any changes are made
-VERSION = "7.1.8"
+VERSION = "7.1.9"
 
 PY_EXTENSIONS = ".py", ".pyw"
 

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`. When extracting Python 2 (`--lang=2`), `except A, e:` continues to bind `e` as an alias, since that is what the syntax means in that version.

python/ql/test/2/extractor-tests/relaxed_except/options

@@ -0,0 +1 @@
+semmle-extractor-options: --lang=2

python/ql/test/2/extractor-tests/relaxed_except/relaxed_except.expected

@@ -0,0 +1,3 @@
+| 6 | ValueError | err (definition) |
+| 12 | ValueError | other (definition) |
+| 18 | ValueError, TypeError | none |

python/ql/test/2/extractor-tests/relaxed_except/relaxed_except.ql

@@ -0,0 +1,26 @@
+/**
+ * The types of each `except` clause, and the name it binds. In Python 2 the
+ * comma form binds a name and has a single type; reading it as a PEP 758 tuple
+ * instead would give two types and no name.
+ */
+
+import python
+
+from ExceptStmt handler, string types, string name
+where
+  types =
+    concat(Expr type |
+      type = handler.getType()
+    |
+      type.toString(), ", " order by type.getLocation().getStartColumn()
+    ) and
+  (
+    exists(Name bound | bound = handler.getName() |
+      bound.isDefinition() and name = bound.getId() + " (definition)"
+      or
+      not bound.isDefinition() and name = bound.getId() + " (use)"
+    )
+    or
+    not exists(handler.getName()) and name = "none"
+  )
+select handler.getLocation().getStartLine(), types, name

python/ql/test/2/extractor-tests/relaxed_except/test.py

@@ -0,0 +1,19 @@
+# When extracting Python 2, `except A, e:` binds `e`. It is not a PEP 758
+# unparenthesized tuple of exception types, which is what the same syntax means
+# from Python 3.14 on.
+try:
+    unlikely()
+except ValueError, err:
+    print err
+
+# `as` means the same thing in every version.
+try:
+    unlikely()
+except ValueError as other:
+    print other
+
+# A parenthesized tuple is several types, and binds nothing.
+try:
+    unlikely()
+except (ValueError, TypeError):
+    pass

python/ql/test/2/query-tests/Imports/unused/UnusedImport.expected

@@ -0,0 +1 @@
+| relaxed_except.py:2:1:2:43 | Import | Import of 'Beta' is not used. |