Fix backward compatibility with pickles before v22.2.0

changelog.d/1085.change.md

@@ -0,0 +1 @@
+Restored ability to unpickle instances pickled before 22.2.0.

src/attr/_make.py

@@ -934,9 +934,15 @@ def slots_setstate(self, state):
             Automatically created by attrs.
             """
             __bound_setattr = _obj_setattr.__get__(self)
-            for name in state_attr_names:
-                if name in state:
-                    __bound_setattr(name, state[name])
+            if isinstance(state, tuple):
+                # Backward compatibility with attrs instances pickled with
+                # attrs versions before v22.2.0 which stored tuples.
+                for name, value in zip(state_attr_names, state):
+                    __bound_setattr(name, value)
+            else:
+                for name in state_attr_names:
+                    if name in state:
+                        __bound_setattr(name, state[name])
 
             # The hash code cache is not included when the object is
             # serialized, but it still needs to be initialized to None to

tests/test_slots.py

@@ -754,13 +754,12 @@ class A:
     c = attr.ib()
 
 
-@pytest.mark.parametrize("cls", [A])
-def test_slots_unpickle_after_attr_removed(cls):
+def test_slots_unpickle_after_attr_removed():
     """
     We don't assign attributes we don't have anymore if the class has
     removed it.
     """
-    a = cls(1, 2, 3)
+    a = A(1, 2, 3)
     a_pickled = pickle.dumps(a)
     a_unpickled = pickle.loads(a_pickled)
     assert a_unpickled == a
@@ -778,12 +777,11 @@ class NEW_A:
         assert not hasattr(new_a, "b")
 
 
-@pytest.mark.parametrize("cls", [A])
-def test_slots_unpickle_after_attr_added(cls, frozen):
+def test_slots_unpickle_after_attr_added(frozen):
     """
     We don't assign attribute we haven't had before if the class has one added.
     """
-    a = cls(1, 2, 3)
+    a = A(1, 2, 3)
     a_pickled = pickle.dumps(a)
     a_unpickled = pickle.loads(a_pickled)
 
@@ -803,3 +801,20 @@ class NEW_A:
         assert new_a.b == 2
         assert new_a.c == 3
         assert not hasattr(new_a, "d")
+
+
+def test_slots_unpickle_is_backward_compatible(frozen):
+    """
+    Ensure object pickled before v22.2.0 can still be unpickled.
+    """
+    a = A(1, 2, 3)
+
+    a_pickled = (
+        b"\x80\x04\x95&\x00\x00\x00\x00\x00\x00\x00\x8c\x10"
+        + a.__module__.encode()
+        + b"\x94\x8c\x01A\x94\x93\x94)\x81\x94K\x01K\x02K\x03\x87\x94b."
+    )
+
+    a_unpickled = pickle.loads(a_pickled)
+
+    assert a_unpickled == a