[2439] Inherit the mock from RRsetCollectionBase

And make the test slightly cleaner by that. No change in functionality,
just rely on assumptions about what breaking of API don't matter little
bit less.

src/lib/python/isc/xfrin/tests/diff_tests.py

@@ -16,7 +16,8 @@
 import isc.log
 import unittest
 from isc.datasrc import ZoneFinder
-from isc.dns import Name, RRset, RRClass, RRType, RRTTL, Rdata, RRsetCollection
+from isc.dns import Name, RRset, RRClass, RRType, RRTTL, Rdata, \
+    RRsetCollectionBase
 from isc.xfrin.diff import Diff, NoSuchZone
 
 class TestError(Exception):
@@ -1087,14 +1088,26 @@ class DiffTest(unittest.TestCase):
             self.__check_find_all_call(diff.find_all, self.__rrset3,
                                        rcode)
 
-    class Collection:
+    class Collection(isc.dns.RRsetCollectionBase):
         '''
-        Our own mock RRsetCollection. We don't use it, just pass it through.
-        This is to implement the below method.
+        Our own mock RRsetCollection. We only pass it through, but we
+        still define an (mostly empty) find method to satisfy the
+        expectations.
         '''
-        # Any idea why python doesn't agree with inheriting from
-        # dns.RRsetCollection?
-        pass
+        def __init__(self):
+            '''
+            Empty init. The base class's __init__ can't be called,
+            so we need to provide our own to shadow it -- and make sure
+            not to call the parent().__init__().
+            '''
+            pass
+
+        def find(self, name, rrclass, rrtype):
+            '''
+            Empty find method. Returns None to each query (pretends
+            the collection is empty. Present mostly for completeness.
+            '''
+            return None
 
     def get_rrset_collection(self):
         '''
@@ -1116,8 +1129,10 @@ class DiffTest(unittest.TestCase):
         self.assertEqual('add', self.__data_operations[0][0])
         # Check the returned one is actually RRsetCollection
         self.assertTrue(collection, self.Collection)
-        # We don't call anything on the collection. It's just the mock from
-        # above, so it wouldn't be any good.
+        # The collection is just the mock from above, so this doesn't do much
+        # testing, but we check that the mock got through and didn't get hurt.
+        self.assertIsNone(collection.find(Name('example.org'), RRClass.IN(),
+                                          RRType.SOA()))
 
 if __name__ == "__main__":
     isc.log.init("bind10")