Added test class for issue 28, which runs OK under Python2.7 but throws an exception under Python3.5 (both on macOS).

As well as adding the test class, this commit turns on DEBUG level logging, which makes it easier to spot the point at which
the behaviour of the two versions diverge.

Author: tim-littlefair

cluster/test/test_hierarchical.py

@@ -231,12 +231,42 @@ def euclidian_distance(a, b):
         result = cl.getlevel(40)
         self.assertIsNotNone(result)
 
+class Issue28TestCase(Py23TestCase):
+    '''
+    Test case to cover the case where the data consist
+    of dictionary keys, and the distance function executes 
+    on the values these keys are associated with in the
+    dictionary, rather than the keys themselves.
+
+    Behaviour for this test case differs between Python2.7
+    and Python3.5: on 2.7 the test behaves as expected, 
+
+    See Github issue #28.
+    '''
+
+    def testIssue28(self):
+        "Issue28 (Hierarchical Clustering)"
 
+        points1D = {
+            'p4' : 5, 'p2' : 6, 'p7' : 10,
+            'p9' : 120, 'p10' : 121, 'p11' : 119,
+        }
+
+        distance_func = lambda a,b : abs(points1D[a]-points1D[b])
+        cl = HierarchicalClustering(list(points1D.keys()), distance_func)
+        result = cl.getlevel(20)
+        self.assertIsNotNone(result)
+    
 if __name__ == '__main__':
+
+    import logging
+
     suite = unittest.TestSuite((
         unittest.makeSuite(HClusterIntegerTestCase),
         unittest.makeSuite(HClusterSmallListTestCase),
         unittest.makeSuite(HClusterStringTestCase),
+        unittest.makeSuite(Issue28TestCase),
     ))
 
+    logging.basicConfig(level=logging.DEBUG)
     unittest.TextTestRunner(verbosity=2).run(suite)