Avoid creating ref cycles

docs/versionhistory.rst

@@ -4,6 +4,11 @@ Version history
 This library adheres to
 `Semantic Versioning 2.0 <https://semver.org/#semantic-versioning-200>`_.
 
+**UNRELEASED**
+
+- Avoid creating reference cycles when type checking unions
+  (`#408 <https://github.com/agronholm/typeguard/pull/408>`_)
+
 **4.1.5** (2023-09-11)
 
 - Fixed ``Callable`` erroneously rejecting a callable that has the requested amount of

src/typeguard/_checkers.py

@@ -395,16 +395,19 @@ def check_union(
     memo: TypeCheckMemo,
 ) -> None:
     errors: dict[str, TypeCheckError] = {}
-    for type_ in args:
-        try:
-            check_type_internal(value, type_, memo)
-            return
-        except TypeCheckError as exc:
-            errors[get_type_name(type_)] = exc
+    try:
+        for type_ in args:
+            try:
+                check_type_internal(value, type_, memo)
+                return
+            except TypeCheckError as exc:
+                errors[get_type_name(type_)] = exc
 
-    formatted_errors = indent(
-        "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
-    )
+        formatted_errors = indent(
+            "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
+        )
+    finally:
+        del errors  # avoid creating ref cycle
     raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
 
 

tests/test_checkers.py

@@ -768,6 +768,35 @@ def test_union_fail(self, annotation, value):
             f"  int: is not an instance of int"
         )
 
+    @pytest.mark.skipif(
+        sys.implementation.name != "cpython",
+        reason="Test relies on CPython's reference counting behavior",
+    )
+    def test_union_reference_leak(self):
+        leaked = True
+
+        class Leak:
+            def __del__(self):
+                nonlocal leaked
+                leaked = False
+
+        def inner1():
+            leak = Leak()  # noqa: F841
+            check_type(b"asdf", Union[str, bytes])
+
+        inner1()
+        assert not leaked
+
+        leaked = True
+
+        def inner2():
+            leak = Leak()  # noqa: F841
+            with pytest.raises(TypeCheckError, match="any element in the union:"):
+                check_type(1, Union[str, bytes])
+
+        inner2()
+        assert not leaked
+
 
 class TestTypevar:
     def test_bound(self):