The set operation difference(X, None) was returning None instead of

returning X, contradicting the docs and common sense.
difference(None, X) continues to return None.

src/BTrees/Interfaces.py

@@ -249,8 +249,11 @@ class IMerge(Interface):
         """Return the keys or items in c1 for which there is no key in
         c2.
 
-        If c1 is None, then None is returned.  If c2 is none, then c1
+        If c1 is None, then None is returned.  If c2 is None, then c1
         is returned.
+
+        If neither c1 nor c2 is None, the output is a Set if c1 is a Set or
+        TreeSet, and is a Bucket if c1 is a Bucket or BTree.
         """
 
     def union(c1, c2):

src/BTrees/SetOpTemplate.c

@@ -16,7 +16,7 @@
  Set operations
  ****************************************************************************/
 
-#define SETOPTEMPLATE_C "$Id: SetOpTemplate.c,v 1.24 2002/06/08 04:41:44 tim_one Exp $\n"
+#define SETOPTEMPLATE_C "$Id: SetOpTemplate.c,v 1.25 2002/06/23 19:36:12 tim_one Exp $\n"
 
 #ifdef INTSET_H
 static int
@@ -346,8 +346,9 @@ difference_m(PyObject *ignored, PyObject *args)
 
   if (o1==Py_None || o2==Py_None)
     {
-      Py_INCREF(Py_None);
-      return Py_None;
+      /* difference(None, X) -> None; difference(X, None) -> X */
+      Py_INCREF(o1);
+      return o1;
     }
 
   return set_operation(o1, o2, 1, -1, 1, 0, 0);

src/BTrees/tests/testSetOps.py

@@ -68,22 +68,16 @@ class SetResult(TestCase):
             C = op(None, None)
             self.assert_(C is None)
 
-        for op in self.union, self.intersection:
+        for op in self.union, self.intersection, self.difference:
             for A in self.As:
                 C = op(A, None)
                 self.assert_(C is A)
 
                 C = op(None, A)
-                self.assert_(C is A)
-
-        # XXX These difference results contradict the docs.  The implementation
-        # XXX is almost certainly wrong, but can we change it?
-        for A in self.As:
-            C = self.difference(A, None)
-            self.assert_(C is None)
-
-            C = self.difference(None, A)
+                if op is self.difference:
                     self.assert_(C is None)
+                else:
+                    self.assert_(C is A)
 
     def testEmptyUnion(self):
         for A in self.As: