Fix ref cycles

docs/versionhistory.rst

@@ -9,6 +9,7 @@ This library adheres to
 - Dropped Python 3.8 support
 - Changed the signature of ``typeguard_ignore()`` to be compatible with
   ``typing.no_type_check()`` (PR by @jolaf)
+- Avoid creating reference cycles when type checking uniontypes and classes
 - Fixed checking of variable assignments involving tuple unpacking
   (`#486 <https://github.com/agronholm/typeguard/pull/486>`_)
 - Fixed ``TypeError`` when checking a class against ``type[Self]``

src/typeguard/_checkers.py

@@ -432,16 +432,20 @@ def check_uniontype(
     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()), "  "
+        )
+    finally:
+        del errors  # avoid creating ref cycle
 
-    formatted_errors = indent(
-        "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
-    )
     raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
 
 
@@ -472,22 +476,25 @@ def check_class(
         check_typevar(value, expected_class, (), memo, subclass_check=True)
     elif get_origin(expected_class) is Union:
         errors: dict[str, TypeCheckError] = {}
-        for arg in get_args(expected_class):
-            if arg is Any:
-                return
+        try:
+            for arg in get_args(expected_class):
+                if arg is Any:
+                    return
 
-            try:
-                check_class(value, type, (arg,), memo)
-                return
-            except TypeCheckError as exc:
-                errors[get_type_name(arg)] = exc
-        else:
-            formatted_errors = indent(
-                "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
-            )
-            raise TypeCheckError(
-                f"did not match any element in the union:\n{formatted_errors}"
-            )
+                try:
+                    check_class(value, type, (arg,), memo)
+                    return
+                except TypeCheckError as exc:
+                    errors[get_type_name(arg)] = exc
+            else:
+                formatted_errors = indent(
+                    "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
+                )
+                raise TypeCheckError(
+                    f"did not match any element in the union:\n{formatted_errors}"
+                )
+        finally:
+            del errors  # avoid creating ref cycle
     elif not issubclass(value, expected_class):  # type: ignore[arg-type]
         raise TypeCheckError(f"is not a subclass of {qualified_name(expected_class)}")
 

tests/test_checkers.py

@@ -816,8 +816,6 @@ def test_union_fail(self, annotation, value):
         reason="Test relies on CPython's reference counting behavior",
     )
     def test_union_reference_leak(self):
-        leaked = True
-
         class Leak:
             def __del__(self):
                 nonlocal leaked
@@ -827,19 +825,63 @@ def inner1():
             leak = Leak()  # noqa: F841
             check_type(b"asdf", Union[str, bytes])
 
+        leaked = True
         inner1()
         assert not leaked
 
+        def inner2():
+            leak = Leak()  # noqa: F841
+            check_type(b"asdf", Union[bytes, str])
+
         leaked = True
+        inner2()
+        assert not leaked
 
-        def inner2():
+        def inner3():
             leak = Leak()  # noqa: F841
             with pytest.raises(TypeCheckError, match="any element in the union:"):
                 check_type(1, Union[str, bytes])
 
+        leaked = True
+        inner3()
+        assert not leaked
+
+    @pytest.mark.skipif(
+        sys.implementation.name != "cpython",
+        reason="Test relies on CPython's reference counting behavior",
+    )
+    @pytest.mark.skipif(sys.version_info < (3, 10), reason="UnionType requires 3.10")
+    def test_uniontype_reference_leak(self):
+        class Leak:
+            def __del__(self):
+                nonlocal leaked
+                leaked = False
+
+        def inner1():
+            leak = Leak()  # noqa: F841
+            check_type(b"asdf", str | bytes)
+
+        leaked = True
+        inner1()
+        assert not leaked
+
+        def inner2():
+            leak = Leak()  # noqa: F841
+            check_type(b"asdf", bytes | str)
+
+        leaked = True
         inner2()
         assert not leaked
 
+        def inner3():
+            leak = Leak()  # noqa: F841
+            with pytest.raises(TypeCheckError, match="any element in the union:"):
+                check_type(1, Union[str, bytes])
+
+        leaked = True
+        inner3()
+        assert not leaked
+
 
 class TestTypevar:
     def test_bound(self):