Support generic parents in include_subclasses strategy (#650)

Resolves #648

The strategy was using parent_cls.__subclasses__() to get the
list of subclasses. In case of generics, this method
is unavailable.

The fix applies sanitizing the cl and getting its origin
class for getting the sublcasses tree.

The class itself remains generic in the tree.

Author: zhukovgreen

HISTORY.md

@@ -46,6 +46,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([#599](https://github.com/python-attrs/cattrs/pull/599))
 - Structuring TypedDicts from invalid inputs now properly raises a {class}`ClassValidationError`.
   ([#615](https://github.com/python-attrs/cattrs/issues/615) [#616](https://github.com/python-attrs/cattrs/pull/616))
+- {func} `cattrs.strategies.include_subclasses` now properly working with generic parent classes.
+  ([#649](https://github.com/python-attrs/cattrs/pull/650))
 - Replace `cattrs.gen.MappingStructureFn` with {class}`cattrs.SimpleStructureHook`.
 - Python 3.13 is now supported.
   ([#543](https://github.com/python-attrs/cattrs/pull/543) [#547](https://github.com/python-attrs/cattrs/issues/547))

src/cattrs/strategies/_subclasses.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import typing
 from gc import collect
 from typing import Any, Callable, TypeVar, Union
 
@@ -11,8 +12,12 @@
 
 
 def _make_subclasses_tree(cl: type) -> list[type]:
+    # get class origin for accessing subclasses (see #648 for more info)
+    cls_origin = typing.get_origin(cl) or cl
     return [cl] + [
-        sscl for scl in cl.__subclasses__() for sscl in _make_subclasses_tree(scl)
+        sscl
+        for scl in cls_origin.__subclasses__()
+        for sscl in _make_subclasses_tree(scl)
     ]
 
 

tests/strategies/test_include_subclasses.py

@@ -9,6 +9,8 @@
 from cattrs.errors import ClassValidationError, StructureHandlerNotFoundError
 from cattrs.strategies import configure_tagged_union, include_subclasses
 
+T = typing.TypeVar("T")
+
 
 @define
 class Parent:
@@ -412,3 +414,21 @@ class NewChild2(NewParent):
 
     with pytest.raises(StructureHandlerNotFoundError):
         include_subclasses(NewParent, genconverter)
+
+
+def test_parents_with_generics(genconverter: Converter):
+    """Ensure proper handling of generic parents #648."""
+
+    @define
+    class GenericParent(typing.Generic[T]):
+        p: T
+
+    @define
+    class Child1G(GenericParent[str]):
+        c: str
+
+    include_subclasses(GenericParent[str], genconverter)
+
+    assert genconverter.structure({"p": 5, "c": 5}, GenericParent[str]) == Child1G(
+        "5", "5"
+    )