Distinguish empty call defaults from comprehension accumulators

src/decompiler.rs

@@ -52942,7 +52942,17 @@ impl<'a> Ctx<'a> {
                     continue
                 }
                 Op::BUILD_LIST | Op::BUILD_SET | Op::BUILD_MAP => {
-                    return ins.arg == 0;
+                    if ins.arg != 0 {
+                        return false;
+                    }
+                    // Modern accumulators are lifted under the iterator
+                    // by SWAP 2. An empty default argument consumed by a
+                    // preceding CALL is not a new comprehension level.
+                    // Preserve the distinct py2 prologue handled above.
+                    return !self.version.at_least(3, 12)
+                        || self.instrs.get(back + 1).map_or(false, |next| {
+                            next.op == Op::SWAP && next.arg == 2
+                        });
                 }
                 o if is_pure_value_op(o)
                     || matches!(

tests/behavior/cases/b_comprehension_default_arguments.py

@@ -0,0 +1,27 @@
+# MIN_VERSION: 3.12
+
+
+def list_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [field for row in rows for field in row.get("values", [])]
+    return result, row, field
+
+
+def dict_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = {field: len(field) for row in rows for field in row.get("values", {})}
+    return result, row, field
+
+
+def nested_list(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [[field for field in row.get("values", [])] for row in rows]
+    return result, row, field
+
+
+print(list_default([{}, {"values": ["a", "bb"]}, {}]))
+print(dict_default([{}, {"values": {"a": 1, "bb": 2}}, {}]))
+print(nested_list([{}, {"values": ["a"]}, {}]))

tests/fixtures/comprehension_default_arguments.py

@@ -0,0 +1,66 @@
+# Compile with each named CPython version; the committed .pyc files are
+# exercised by the existing Rust integration suite.
+
+# Python 3.12+ inlines these comprehensions; 3.11 is the legacy control.
+
+def list_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [field for row in rows for field in row.get("values", [])]
+    return result, row, field
+
+
+def set_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = {field for row in rows for field in row.get("values", {})}
+    return result, row, field
+
+
+def dict_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = {field: len(field) for row in rows for field in row.get("values", {})}
+    return result, row, field
+
+
+def nested_list(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [[field for field in row.get("values", [])] for row in rows]
+    return result, row, field
+
+
+def nested_set(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [{field for field in row.get("values", {})} for row in rows]
+    return result, row, field
+
+
+def nested_dict(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [{field: len(field) for field in row.get("values", {})} for row in rows]
+    return result, row, field
+
+
+def subscript(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [field for row in rows for field in row["values"]]
+    return result, row, field
+
+
+def no_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [field for row in rows for field in row.get("values")]
+    return result, row, field
+
+
+def tuple_default(rows):
+    row = "saved-row"
+    field = "saved-field"
+    result = [field for row in rows for field in row.get("values", ())]
+    return result, row, field

tests/integration.rs

@@ -169,3 +169,27 @@ fn py314_hello() {
         "py3.14 hello",
     );
 }
+
+#[test]
+fn empty_defaults_keep_the_comprehension_level() {
+    // PEP 709 inlining starts in 3.12; retain the 3.11 compatibility case.
+    for version in ["3.11", "3.12", "3.13", "3.14"] {
+        let (src, clean) = decompile_fixture(&format!("comprehension_default_arguments.{version}.pyc"));
+        assert!(clean, "Python {version}: output marked incomplete:\n{src}");
+        for (name, expression) in [
+            ("list_default", "[field for row in rows for field in row.get('values', [])]"),
+            ("set_default", "{field for row in rows for field in row.get('values', {})}"),
+            ("dict_default", "{field: len(field) for row in rows for field in row.get('values', {})}"),
+            ("nested_list", "[[field for field in row.get('values', [])] for row in rows]"),
+            ("nested_set", "[{field for field in row.get('values', {})} for row in rows]"),
+            ("nested_dict", "[{field: len(field) for field in row.get('values', {})} for row in rows]"),
+            ("subscript", "[field for row in rows for field in row['values']]"),
+            ("no_default", "[field for row in rows for field in row.get('values')]"),
+            ("tuple_default", "[field for row in rows for field in row.get('values', ())]"),
+        ] {
+            let expected = format!(
+                "def {name}(rows):\n    row = 'saved-row'\n    field = 'saved-field'\n    result = {expression}\n    return result, row, field");
+            assert_contains(&src, &[&expected], version);
+        }
+    }
+}