weightedIntersection must work even when merging is impossible (e.g. two sets).

BTrees/_base.py

@@ -1286,7 +1286,6 @@ def weightedIntersection(set_type, o1, o2, w1=1, w2=1):
     MERGE_DEFAULT = getattr(o1, 'MERGE_DEFAULT', None)
     i1 = _SetIteration(o1, True, MERGE_DEFAULT)
     i2 = _SetIteration(o2, True, MERGE_DEFAULT)
-    result = o1._mapping_type()
     MERGE = getattr(o1, 'MERGE', None)
     if MERGE is None and i1.useValues and i2.useValues:
         raise TypeError("invalid set operation")
@@ -1300,15 +1299,23 @@ def weightedIntersection(set_type, o1, o2, w1=1, w2=1):
                 raise TypeError("invalid set operation")
         else:
             raise TypeError("invalid set operation")
+    _merging = i1.useValues or i2.useValues
+    if _merging:
+        result = o1._mapping_type()
         def copy(i, w):
             result._keys.append(i.key)
             result._values.append(MERGE_WEIGHT(i.value, w))
+    else:
+        result = o1._set_type()
+        def copy(i, w):
+            result._keys.append(i.key)
     while i1.active and i2.active:
         cmp_ = cmp(i1.key, i2.key)
         if cmp_ < 0:
             i1.advance()
         elif cmp_ == 0:
             result._keys.append(i1.key)
+            if _merging:
                 result._values.append(MERGE(i1.value, w1, i2.value, w2))
             i1.advance()
             i2.advance()

BTrees/tests/test__base.py

@@ -2653,6 +2653,14 @@ class Test_weightedUnion(unittest.TestCase, _SetObBase):
         self.assertEqual(result['d'], 33)
         self.assertEqual(result['e'], 11)
 
+    def test_w_lhs_Set_rhs_Set(self):
+        from BTrees.IIBTree import IISetPy
+        lhs = IISetPy([1, 2, 3])
+        rhs = IISetPy([1, 4])
+        weight, result = self._callFUT(lhs.__class__, lhs, rhs)
+        self.assertEqual(weight, 1)
+        self.assertEqual(list(result), [1, 2, 3, 4])
+
 
 class Test_weightedIntersection(unittest.TestCase, _SetObBase):
 
@@ -2695,6 +2703,14 @@ class Test_weightedIntersection(unittest.TestCase, _SetObBase):
         self.assertEqual(list(result), ['a'])
         self.assertEqual(result['a'], 23)
 
+    def test_w_lhs_Set_rhs_Set(self):
+        from BTrees.IIBTree import IISetPy
+        lhs = IISetPy([1, 2, 3])
+        rhs = IISetPy([1, 4])
+        weight, result = self._callFUT(lhs.__class__, lhs, rhs)
+        self.assertEqual(weight, 2)
+        self.assertEqual(list(result), [1])
+
 
 class Test_multiunion(unittest.TestCase, _SetObBase):