Do not untructure the Singleton attrs.NOTHING at all (identity) instead of to 1 (int) (attrs >= 22.2 problem)

HISTORY.md

@@ -28,6 +28,10 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
 - For {class}`cattrs.errors.StructureHandlerNotFoundError` and {class}`cattrs.errors.ForbiddenExtraKeysError` 
   correctly set {attr}`BaseException.args` in `super()` and hence make them pickable. 
   ([#666](https://github.com/python-attrs/cattrs/pull/666))
+- Do not Unstructure the faulty state {data}`attrs.NOTHING` instead of to `1` (int). The unstructure to `1` was caused
+  by a change in attrs 22.2 that {data}`attrs.NOTHING` is internally represented as an integer {class}`enum.Enum`, 
+  but as it is logically just a Singleton we apply now the identity-function on unstructuring it. 
+  ([#667](https://github.com/python-attrs/cattrs/pull/667))
 
 ## 25.1.1 (2025-06-04)
 

src/cattrs/converters.py

@@ -11,6 +11,7 @@
 from pathlib import Path
 from typing import Any, Optional, Tuple, TypeVar, overload
 
+from attr._make import _Nothing
 from attrs import Attribute, resolve_types
 from attrs import has as attrs_has
 from typing_extensions import Self
@@ -223,7 +224,7 @@ def __init__(
             unstructure_fallback_factory, self
         )
         self._unstructure_func.register_cls_list(
-            [(bytes, identity), (str, identity), (Path, str)]
+            [(bytes, identity), (str, identity), (Path, str), (_Nothing, identity)]
         )
         self._unstructure_func.register_func_list(
             [

tests/test_enums.py

@@ -1,5 +1,6 @@
 """Tests for enums."""
 
+from attrs import NOTHING
 from hypothesis import given
 from hypothesis.strategies import data, sampled_from
 from pytest import raises
@@ -29,3 +30,17 @@ def test_enum_failure(enum):
         converter.structure("", type)
 
     assert exc_info.value.args[0] == f" not in literal {type!r}"
+
+
+def test_nothing_from_attrs():
+    """Test that `NOTHING` from attrs does not unstructure to `1` (int), but remains `NOTHING`."""
+    converter = BaseConverter()
+
+    assert (
+        converter.unstructure(NOTHING) != 1
+    ), "NOTHING should not unstructure to 1 (int)."
+    assert not isinstance(converter.unstructure(NOTHING), int)
+    assert not converter.unstructure(
+        NOTHING
+    )  # bool(NOTHING) should be False although `bool(1)` is True
+    assert converter.unstructure(NOTHING) is NOTHING

tests/test_errors.py

@@ -18,9 +18,9 @@
     "err_cls, err_args",
     [
         (StructureHandlerNotFoundError, ("Structure Message", int)),
-        (ForbiddenExtraKeysError, ("Forbidden Message", int, {"foo", "bar"})),
-        (ForbiddenExtraKeysError, ("", str, {"foo", "bar"})),
-        (ForbiddenExtraKeysError, (None, list, {"foo", "bar"})),
+        (ForbiddenExtraKeysError, ("Forbidden Message", int, {"foo"})),
+        (ForbiddenExtraKeysError, ("", str, {"foo"})),
+        (ForbiddenExtraKeysError, (None, list, {"foo"})),
         (
             BaseValidationError,
             ("BaseValidation Message", [ValueError("Test BaseValidation")], int),