diff -r 05acdf8e1f23 -r a45e44d76c81 tests/test-dicthelpers.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-dicthelpers.py	Mon Mar 25 17:40:39 2013 -0700
@@ -0,0 +1,53 @@
+from mercurial.dicthelpers import diff, join
+import unittest
+import silenttestrunner
+
+class testdicthelpers(unittest.TestCase):
+    def test_dicthelpers(self):
+        # empty dicts
+        self.assertEqual(diff({}, {}), {})
+        self.assertEqual(join({}, {}), {})
+
+        d1 = {}
+        d1['a'] = 'foo'
+        d1['b'] = 'bar'
+        d1['c'] = 'baz'
+
+        # same identity
+        self.assertEqual(diff(d1, d1), {})
+        self.assertEqual(join(d1, d1), {'a': ('foo', 'foo'),
+                                        'b': ('bar', 'bar'),
+                                        'c': ('baz', 'baz')})
+
+        # vs empty
+        self.assertEqual(diff(d1, {}), {'a': ('foo', None),
+                                        'b': ('bar', None),
+                                        'c': ('baz', None)})
+        self.assertEqual(diff(d1, {}), {'a': ('foo', None),
+                                        'b': ('bar', None),
+                                        'c': ('baz', None)})
+
+        d2 = {}
+        d2['a'] = 'foo2'
+        d2['b'] = 'bar'
+        d2['d'] = 'quux'
+
+        self.assertEqual(diff(d1, d2), {'a': ('foo', 'foo2'),
+                                        'c': ('baz', None),
+                                        'd': (None, 'quux')})
+        self.assertEqual(join(d1, d2), {'a': ('foo', 'foo2'),
+                                        'b': ('bar', 'bar'),
+                                        'c': ('baz', None),
+                                        'd': (None, 'quux')})
+
+        # with default argument
+        self.assertEqual(diff(d1, d2, 123), {'a': ('foo', 'foo2'),
+                                             'c': ('baz', 123),
+                                             'd': (123, 'quux')})
+        self.assertEqual(join(d1, d2, 456), {'a': ('foo', 'foo2'),
+                                             'b': ('bar', 'bar'),
+                                             'c': ('baz', 456),
+                                             'd': (456, 'quux')})
+
+if __name__ == '__main__':
+    silenttestrunner.main(__name__)