Fix check for duplicate atom labels.

Raise IndexError when atom cannot be uniquely looked up
by its string label.

Author: pavoljuhas

diffpy/Structure/structure.py

@@ -395,11 +395,11 @@ def __getitem__(self, idx):
             return rv
         # here we need to resolve at least one string label
         # build a map of labels to indices and mark duplicate labels
-        duplicate = ()
+        duplicate = object()
         labeltoindex = {}
         for i, a in enumerate(self):
             labeltoindex[a.label] = (
-                    a.label in labeltoindex and duplicate or i)
+                duplicate if a.label in labeltoindex else i)
         def _resolveindex(aid):
             aid1 = aid
             if type(aid) is str:

diffpy/Structure/tests/TestStructure.py

@@ -256,6 +256,11 @@ def test___getitem__(self):
         self.failUnless(cdse[0] is cdse['Hohenzollern'])
         self.assertEqual([cdse[0], cdse[3], cdse[1]],
                 cdse['Hohenzollern', 3:0:-2].tolist())
+        stru.label = ['A', 'B']
+        self.assertTrue(stru[0] is stru['A'])
+        self.assertTrue(stru[1] is stru['B'])
+        stru[1].label = 'A'
+        self.assertRaises(IndexError, stru.__getitem__, 'A')
         return