Fixed frozen and cache_hash incompatibility

Fixes GH issue #611.

changelog.d/611.change.rst

@@ -0,0 +1 @@
+Fixed serialization of classes with ``frozen=True`` and ``cache_hash=True``, which has been broken since 19.1.0.

src/attr/_make.py

@@ -537,9 +537,17 @@ def _patch_original_class(self):
                     "See https://github.com/python-attrs/attrs/issues/494 ."
                 )
 
-            def cache_hash_set_state(chss_self, _):
-                # clear hash code cache
-                setattr(chss_self, _hash_cache_field, None)
+            # Clears the cached hash state on serialization; for frozen
+            # classes we need to bypass the class's setattr method.
+            if self._frozen:
+
+                def cache_hash_set_state(chss_self, _):
+                    object.__setattr__(chss_self, _hash_cache_field, None)
+
+            else:
+
+                def cache_hash_set_state(chss_self, _):
+                    setattr(chss_self, _hash_cache_field, None)
 
             cls.__setstate__ = cache_hash_set_state
 

tests/test_make.py

@@ -1466,6 +1466,17 @@ class C2(C):
 
         assert [C2] == C.__subclasses__()
 
+    def test_cache_hash_with_frozen_serializes(self):
+        """
+        Frozen classes with cache_hash should be serializable.
+        """
+
+        @attr.s(cache_hash=True, frozen=True)
+        class C(object):
+            pass
+
+        copy.deepcopy(C)
+
 
 class TestMakeOrder:
     """