Preserve SortedSet state when symmetric difference fails

src/sortedcontainers/sortedset.py

@@ -599,9 +599,15 @@ def symmetric_difference_update(self, other):
         """
         _set = self._set
         _list = self._list
-        _set.symmetric_difference_update(other)
-        _list.clear()
-        _list.update(_set)
+        candidate_set = _set.symmetric_difference(other)
+        candidate_list = SortedList(key=self._key)
+        candidate_list._reset(_list._load)
+        candidate_list.update(candidate_set)
+        # Validate keys and comparisons before changing either backing container.
+        # Preserve their identities because public methods are bound to them.
+        _set.clear()
+        _set.update(candidate_set)
+        _list.__dict__.update(candidate_list.__dict__)
         return self
 
     __ixor__ = symmetric_difference_update

tests/test_sortedset_symmetric_difference_failure.py

@@ -0,0 +1,33 @@
+import pytest
+
+from sortedcontainers import SortedSet
+
+
+@pytest.mark.parametrize('key', [None, int])
+@pytest.mark.parametrize('operation', ['method', 'ixor'])
+def test_failed_symmetric_difference_preserves_membership_and_index(key, operation):
+    values = SortedSet([1, 2, 3], key=key)
+    values._reset(7)
+    bisect_left = values.bisect_left
+    isdisjoint = values.isdisjoint
+    incoming = iter([2, 4, 'invalid'])
+
+    with pytest.raises((TypeError, ValueError)):
+        if operation == 'method':
+            values.symmetric_difference_update(incoming)
+        else:
+            values ^= incoming
+
+    assert list(values) == [1, 2, 3]
+    assert len(values) == 3
+    assert 'invalid' not in values
+    assert bisect_left(2) == 1
+    assert isdisjoint([4])
+    values._check()
+
+    assert values.symmetric_difference_update(iter([2, 4])) is values
+    assert list(values) == [1, 3, 4]
+    assert bisect_left(4) == 2
+    assert not isdisjoint([4])
+    assert values._list._load == 7
+    values._check()