Ignore __weakref__ in __setstate__ & __getstate__

Fixes #311

changelog.d/311.change.rst

@@ -0,0 +1 @@
+In slots classes, ``__getstate__`` and ``__setstate__`` now ignore the ``__weakref__`` attribute.

changelog.d/324.change.rst

@@ -0,0 +1 @@
+In slots classes, ``__getstate__`` and ``__setstate__`` now ignore the ``__weakref__`` attribute.

src/attr/_make.py

@@ -443,20 +443,26 @@ def _create_slots_class(self):
         if qualname is not None:
             cd["__qualname__"] = qualname
 
-        attr_names = tuple(self._attr_names)
+        # __weakref__ is not writable.
+        state_attr_names = tuple(
+            an for an in self._attr_names if an != "__weakref__"
+        )
 
         def slots_getstate(self):
             """
             Automatically created by attrs.
             """
-            return tuple(getattr(self, name) for name in attr_names)
+            return tuple(
+                getattr(self, name)
+                for name in state_attr_names
+            )
 
         def slots_setstate(self, state):
             """
             Automatically created by attrs.
             """
             __bound_setattr = _obj_setattr.__get__(self, Attribute)
-            for name, value in zip(attr_names, state):
+            for name, value in zip(state_attr_names, state):
                 __bound_setattr(name, value)
 
         # slots and frozen require __getstate__/__setstate__ to work

tests/test_make.py

@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, division, print_function
 
+import copy
 import inspect
 import itertools
 import sys
@@ -1054,3 +1055,16 @@ def fake_meth(self):
 
         assert "42" == rv.__module__ == fake_meth.__module__
         assert "23" == rv.__qualname__ == fake_meth.__qualname__
+
+    def test_weakref_setstate(self):
+        """
+        __weakref__ is not set on in setstate because it's not writable in
+        slots classes.
+        """
+        @attr.s(slots=True)
+        class C(object):
+            __weakref__ = attr.ib(
+                init=False, hash=False, repr=False, cmp=False
+            )
+
+        assert C() == copy.deepcopy(C())