fix not-equal comparison of `SiteFamily`s

kwant/builder.py

@@ -137,7 +137,7 @@ class SiteFamily(object):
         try:
             return self.canonical_repr != other.canonical_repr
         except AttributeError:
-            return False
+            return True
 
     @abc.abstractmethod
     def normalize_tag(self, tag):

kwant/tests/test_builder.py

@@ -9,7 +9,7 @@
 from __future__ import division
 import warnings
 from random import Random
-from nose.tools import assert_raises
+from nose.tools import assert_raises, assert_not_equal
 from numpy.testing import assert_equal, assert_almost_equal
 import tinyarray as ta
 import kwant
@@ -103,6 +103,12 @@ def test_site_families():
     assert_equal(sys[ofam(1)], 123)
     assert_raises(KeyError, sys.__getitem__, yafam(1))
 
+    # test site families compare equal/not-equal
+    assert_equal(fam, ofam)
+    assert_not_equal(fam, yafam)
+    assert_not_equal(fam, None)
+    assert_not_equal(fam, 'a')
+
 
 class VerySimpleSymmetry(builder.Symmetry):
     def __init__(self, period):