Clarify that Set._from_iterable is not required to be a classmethod. (GH-23272)
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 7c7f865..150c2a1 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -1559,6 +1559,62 @@ def assertSameSet(self, s1, s2):
# coerce both to a real set then check equality
self.assertSetEqual(set(s1), set(s2))
+ def test_Set_from_iterable(self):
+ """Verify _from_iterable overriden to an instance method works."""
+ class SetUsingInstanceFromIterable(MutableSet):
+ def __init__(self, values, created_by):
+ if not created_by:
+ raise ValueError(f'created_by must be specified')
+ self.created_by = created_by
+ self._values = set(values)
+
+ def _from_iterable(self, values):
+ return type(self)(values, 'from_iterable')
+
+ def __contains__(self, value):
+ return value in self._values
+
+ def __iter__(self):
+ yield from self._values
+
+ def __len__(self):
+ return len(self._values)
+
+ def add(self, value):
+ self._values.add(value)
+
+ def discard(self, value):
+ self._values.discard(value)
+
+ impl = SetUsingInstanceFromIterable([1, 2, 3], 'test')
+
+ actual = impl - {1}
+ self.assertIsInstance(actual, SetUsingInstanceFromIterable)
+ self.assertEqual('from_iterable', actual.created_by)
+ self.assertEqual({2, 3}, actual)
+
+ actual = impl | {4}
+ self.assertIsInstance(actual, SetUsingInstanceFromIterable)
+ self.assertEqual('from_iterable', actual.created_by)
+ self.assertEqual({1, 2, 3, 4}, actual)
+
+ actual = impl & {2}
+ self.assertIsInstance(actual, SetUsingInstanceFromIterable)
+ self.assertEqual('from_iterable', actual.created_by)
+ self.assertEqual({2}, actual)
+
+ actual = impl ^ {3, 4}
+ self.assertIsInstance(actual, SetUsingInstanceFromIterable)
+ self.assertEqual('from_iterable', actual.created_by)
+ self.assertEqual({1, 2, 4}, actual)
+
+ # NOTE: ixor'ing with a list is important here: internally, __ixor__
+ # only calls _from_iterable if the other value isn't already a Set.
+ impl ^= [3, 4]
+ self.assertIsInstance(impl, SetUsingInstanceFromIterable)
+ self.assertEqual('test', impl.created_by)
+ self.assertEqual({1, 2, 4}, impl)
+
def test_Set_interoperability_with_real_sets(self):
# Issue: 8743
class ListSet(Set):