At Tim Peter's suggestion, propagated GvR's binary operator changes to

the inplace operators.  The strategy is to have the operator overloading
code do the work and then to define equivalent method calls which rely on
the operators.  The changes facilitate proper application of TypeError
and NonImplementedErrors.

Added corresponding tests to the test suite to make sure both the operator
and method call versions get exercised.

Add missing tests for difference_update().

Lib/test/test_sets.py

@@ -219,6 +219,10 @@ class TestUpdateOps(unittest.TestCase):
         self.set |= Set([8])
         assert self.set == Set([2, 4, 6, 8]), "Non-overlapping union"
 
+    def test_union_method_call(self):
+        self.set.union_update(Set([3, 4, 5]))
+        assert self.set == Set([2, 3, 4, 5, 6]), "Union method call"
+
     def test_intersection_subset(self):
         self.set &= Set((2, 4))
         assert self.set == Set((2, 4)), "Subset intersection"
@@ -235,6 +239,10 @@ class TestUpdateOps(unittest.TestCase):
         self.set &= Set([8])
         assert self.set == empty_set, "Non-overlapping intersection"
 
+    def test_intersection_method_call(self):
+        self.set.intersection_update(Set([3, 4, 5]))
+        assert self.set == Set([4]), "Intersection method call"
+
     def test_sym_difference_subset(self):
         self.set ^= Set((2, 4))
         assert self.set == Set([6]), "Subset symmetric difference"
@@ -251,6 +259,30 @@ class TestUpdateOps(unittest.TestCase):
         self.set ^= Set([8])
         assert self.set == Set([2, 4, 6, 8]), "Non-overlapping symmetric difference"
 
+    def test_sym_difference_method_call(self):
+        self.set.symmetric_difference_update(Set([3, 4, 5]))
+        assert self.set == Set([2, 3, 5, 6]), "Symmetric difference method call"
+
+    def test_difference_subset(self):
+        self.set -= Set((2, 4))
+        assert self.set == Set([6]), "Subset difference"
+
+    def test_difference_superset(self):
+        self.set -= Set((2, 4, 6, 8))
+        assert self.set == Set([]), "Superset difference"
+
+    def test_difference_overlap(self):
+        self.set -= Set((3, 4, 5))
+        assert self.set == Set([2, 6]), "Overlapping difference"
+
+    def test_difference_non_overlap(self):
+        self.set -= Set([8])
+        assert self.set == Set([2, 4, 6]), "Non-overlapping difference"
+
+    def test_difference_method_call(self):
+        self.set.difference_update(Set([3, 4, 5]))
+        assert self.set == Set([2, 6]), "Difference method call"
+
 #==============================================================================
 
 class TestMutate(unittest.TestCase):