Bug 1052526 - Treat appending None to List as empty list; r=glandium

As a special exception, we change List's + and += operators to treat
"+ None" and "+= None" as "+ []" and "+= []" respectively.

This is a hack to make moz.build files simpler so they don't have to
perform "is x" checks before appending x.

While I was here, I fixed the implementation of List.__add__ to return a
List instead of list.

--HG--
extra : rebase_source : e61db4be4cf8144b092609d3baf84c372b9cf9a0
extra : amend_source : dfabc00765582764426f7cf74e79354a0bfb0824

python/mozbuild/mozbuild/test/test_containers.py

@@ -57,15 +57,36 @@ class TestList(unittest.TestCase):
         test = List([1, 2, 3])
 
         test += [4, 5, 6]
-
+        self.assertIsInstance(test, List)
         self.assertEqual(test, [1, 2, 3, 4, 5, 6])
 
+        test = test + [7, 8]
+        self.assertIsInstance(test, List)
+        self.assertEqual(test, [1, 2, 3, 4, 5, 6, 7, 8])
+
     def test_add_string(self):
         test = List([1, 2, 3])
 
         with self.assertRaises(ValueError):
             test += 'string'
 
+    def test_none(self):
+        """As a special exception, we allow None to be treated as an empty
+        list."""
+        test = List([1, 2, 3])
+
+        test += None
+        self.assertEqual(test, [1, 2, 3])
+
+        test = test + None
+        self.assertIsInstance(test, List)
+        self.assertEqual(test, [1, 2, 3])
+
+        with self.assertRaises(ValueError):
+            test += False
+
+        with self.assertRaises(ValueError):
+            test = test + False
 
 if __name__ == '__main__':
     main()

python/mozbuild/mozbuild/util.py

@@ -249,12 +249,16 @@ class List(list):
         return list.__setslice__(self, i, j, sequence)
 
     def __add__(self, other):
+        # Allow None is a special case because it makes undefined variable
+        # references in moz.build behave better.
+        other = [] if other is None else other
         if not isinstance(other, list):
             raise ValueError('Only lists can be appended to lists.')
 
-        return list.__add__(self, other)
+        return List(list.__add__(self, other))
 
     def __iadd__(self, other):
+        other = [] if other is None else other
         if not isinstance(other, list):
             raise ValueError('Only lists can be appended to lists.')