fix MultiStrategyDispatch to work with new GenConverter and attrs inheritance

HISTORY.rst

@@ -7,6 +7,8 @@ History
 * ``converter.unstructure`` now supports an optional parameter, `unstructure_as`, which can be used to unstructure something as a different type. Useful for unions.
 * Improve support for union un/structuring hooks. Flesh out docs for advanced union handling.
   (`#115 <https://github.com/Tinche/cattrs/pull/115>`_)
+* Fix `GenConverter` behavior with inheritance hierarchies of `attrs` classes.
+  (`#117 <https://github.com/Tinche/cattrs/pull/117>`_) (`#116 <https://github.com/Tinche/cattrs/issues/116>`_)
 
 1.1.2 (2020-11-29)
 ------------------

src/cattr/converters.py

@@ -508,7 +508,9 @@ def unstructure_attrs_asdict(self, obj: Any) -> Dict[str, Any]:
             omit_if_default=self.omit_if_default,
             **attrib_overrides
         )
-        self.register_unstructure_hook(obj.__class__, h)
+        self._unstructure_func.register_cls_list(
+            [(obj.__class__, h)], no_singledispatch=True
+        )
         return h(obj)
 
     def structure_attrs_fromdict(
@@ -524,5 +526,8 @@ def structure_attrs_fromdict(
             if a.type in self.type_overrides
         }
         h = make_dict_structure_fn(cl, self, **attrib_overrides)
-        self.register_structure_hook(cl, h)
+        self._structure_func.register_cls_list(
+            [(cl, h)], no_singledispatch=True
+        )
+        # only direct dispatch so that subclasses get separately generated
         return h(obj, cl)

src/cattr/multistrategy_dispatch.py

@@ -15,34 +15,50 @@ class _DispatchNotFound(object):
 class MultiStrategyDispatch(object):
     """
     MultiStrategyDispatch uses a
-    combination of FunctionDispatch and singledispatch.
+    combination of exact-match dispatch, singledispatch, and FunctionDispatch.
 
-    singledispatch is attempted first. If nothing is
-    registered for singledispatch, or an exception occurs,
+    Exact match dispatch is attempted first, based on a direct
+    lookup of the exact class type, if the hook was registered to avoid singledispatch.
+    singledispatch is attempted next - it will handle subclasses of base classes using MRO
+    If nothing is registered for singledispatch, or an exception occurs,
     the FunctionDispatch instance is then used.
     """
 
-    __slots__ = ("_function_dispatch", "_single_dispatch", "dispatch")
+    __slots__ = (
+        "_direct_dispatch",
+        "_function_dispatch",
+        "_single_dispatch",
+        "dispatch",
+    )
 
     def __init__(self, fallback_func):
+        self._direct_dispatch = {}
         self._function_dispatch = FunctionDispatch()
         self._function_dispatch.register(lambda _: True, fallback_func)
         self._single_dispatch = singledispatch(_DispatchNotFound)
         self.dispatch = lru_cache(maxsize=None)(self._dispatch)
 
     def _dispatch(self, cl):
         try:
+            direct_dispatch = self._direct_dispatch.get(cl)
+            if direct_dispatch is not None:
+                return direct_dispatch
             dispatch = self._single_dispatch.dispatch(cl)
             if dispatch is not _DispatchNotFound:
                 return dispatch
         except Exception:
             pass
         return self._function_dispatch.dispatch(cl)
 
-    def register_cls_list(self, cls_and_handler):
-        """ register a class to singledispatch """
+    def register_cls_list(
+        self, cls_and_handler, no_singledispatch: bool = False
+    ):
+        """ register a class to direct or singledispatch """
         for cls, handler in cls_and_handler:
-            self._single_dispatch.register(cls, handler)
+            if no_singledispatch:
+                self._direct_dispatch[cls] = handler
+            else:
+                self._single_dispatch.register(cls, handler)
         self.dispatch.cache_clear()
 
     def register_func_list(self, func_and_handler):

tests/test_genconverter_inheritance.py

@@ -0,0 +1,20 @@
+from cattr.converters import GenConverter
+import attr
+
+
+def test_inheritance():
+    @attr.s(auto_attribs=True)
+    class A:
+        i: int
+
+    @attr.s(auto_attribs=True)
+    class B(A):
+        j: int
+
+    converter = GenConverter()
+
+    # succeeds
+    assert A(1) == converter.structure(dict(i=1), A)
+
+    # fails
+    assert B(1, 2) == converter.structure(dict(i=1, j=2), B)