Working set fix (#722)

* Add failing test

* Implement the fix

* Document the test

---------

Co-authored-by: Matthieu Melot <matthieu.melot@gmail.com>

Author: mm-andritz

HISTORY.md

@@ -11,6 +11,11 @@ The third number is for emergencies when we need to start branches for older rel
 
 Our backwards-compatibility policy can be found [here](https://github.com/python-attrs/cattrs/blob/main/.github/SECURITY.md).
 
+## NEXT (UNRELEASED)
+
+- Fix an `AttributeError` in `cattrs` internals that could be triggered by using the `include_subclasses` strategy in a `structure_hook_factory`
+  ([#721](https://github.com/python-attrs/cattrs/issues/721), [#722](https://github.com/python-attrs/cattrs/pull/722))
+
 ## 26.1.0 (2026-02-18)
 
 - Add the {mod}`tomllib <cattrs.preconf.tomllib>` preconf converter.

src/cattrs/strategies/_subclasses.py

@@ -191,6 +191,11 @@ def _include_subclasses_with_union_strategy(
 
     original_unstruct_hooks = {}
     original_struct_hooks = {}
+
+    original_working_set = None
+    if hasattr(already_generating, "working_set"):
+        original_working_set = already_generating.working_set.copy()
+
     for cl in union_classes:
         # In the first pass, every class gets its own unstructure function according to
         # the overrides.
@@ -209,6 +214,9 @@ def _include_subclasses_with_union_strategy(
         original_unstruct_hooks[cl] = unstruct_hook
         original_struct_hooks[cl] = struct_hook
 
+    if original_working_set is not None:
+        already_generating.working_set = original_working_set
+
     # Now that's done, we can register all the hooks and generate the
     # union handler. The union handler needs them.
     final_union = Union[union_classes]  # type: ignore

tests/strategies/test_include_subclasses.py

@@ -6,10 +6,11 @@
 from typing import Any
 
 import pytest
-from attrs import define
+from attrs import define, frozen, has
 
 from cattrs import Converter, override
 from cattrs.errors import ClassValidationError, StructureHandlerNotFoundError
+from cattrs.gen import make_dict_structure_fn
 from cattrs.strategies import configure_tagged_union, include_subclasses
 
 from .._compat import is_py311_plus
@@ -536,3 +537,91 @@ class Sub(Mid1, Mid2):
     assert genconverter.structure({"_type": "Sub"}, Base) == Sub()
     assert genconverter.structure({"_type": "Mid1"}, Base) == Mid1()
     assert genconverter.structure({"_type": "Mid2"}, Base) == Mid2()
+
+
+def test_subclasses_in_struct_factory():
+    """
+    Check the structuring does not fail with an attribute error when include_subclasses
+    is called within a structure_hook_factory on a complex class tree involving
+    subclasses several levels deep (#721)
+    """
+
+    @frozen
+    class SubA:
+        id: int
+        sub_a: str
+
+    @frozen
+    class SubA1(SubA):
+        pass
+
+    @frozen
+    class A:
+        """Base class"""
+
+        s: SubA
+
+    @frozen
+    class A1(A):
+        a1: int
+
+    @frozen
+    class A2(A):
+        a2: int
+
+    @frozen
+    class B:
+        id: int
+        b: str
+
+    @frozen
+    class Container1:
+        id: int
+        a: A
+        b: B
+
+    @frozen
+    class Container2:
+        id: int
+        c: Container1
+        foo: str
+
+    def struct_hook_factory(cl, converter: Converter):
+        struct_hook = make_dict_structure_fn(cl, converter)
+        if not cl.__subclasses__():
+            converter.register_structure_hook(cl, struct_hook)
+
+        else:
+
+            def cls_is_cl(cls, _cl=cl):
+                return cls is _cl
+
+            converter.register_structure_hook_func(cls_is_cl, struct_hook)
+            union_strategy = partial(configure_tagged_union, tag_name="type")
+            include_subclasses(cl, converter, union_strategy=union_strategy)
+
+        return converter.get_structure_hook(cl)
+
+    converter = Converter()
+    converter.register_structure_hook_factory(has, struct_hook_factory)
+
+    unstructured = {
+        "id": 0,
+        "c": {
+            "id": 1,
+            "a": {
+                "type": "A1",
+                "s": {"type": "SubA1", "id": 2, "sub_a": "a"},
+                "a1": 42,
+            },
+            "b": {"id": 3, "b": "hello"},
+        },
+        "foo": "world",
+    }
+    res = converter.structure(unstructured, Container2)
+
+    assert res == Container2(
+        id=0,
+        c=Container1(id=1, a=A1(s=SubA1(id=2, sub_a="a"), a1=42), b=B(id=3, b="hello")),
+        foo="world",
+    )