Regression test and patch for ticket #20924.

django/utils/functional.py

@@ -270,6 +270,14 @@ class LazyObject(object):
     def __delitem__(self, key):
         del self[key]
 
+    @new_method_proxy
+    def __len__(self):
+        return len(self)
+
+    @new_method_proxy
+    def __contains__(self, key):
+        return key in self
+
 
 # Workaround for http://bugs.python.org/issue12370
 _super = super

tests/utils_tests/test_simplelazyobject.py

@@ -136,6 +136,9 @@ class TestUtilsSimpleLazyObject(TestCase):
         self.assertEqual(lazydict['one'], 1)
         lazydict['one'] = -1
         self.assertEqual(lazydict['one'], -1)
+        self.assertTrue('one' in lazydict)
+        self.assertFalse('two' in lazydict)
+        self.assertEqual(len(lazydict), 1)
         del lazydict['one']
         with self.assertRaises(KeyError):
             lazydict['one']
@@ -183,3 +186,13 @@ class TestUtilsSimpleLazyObject(TestCase):
 
             # This would fail with "TypeError: expected string or Unicode object, NoneType found".
             pickled = cPickle.dumps(x)
+
+    def test_list_set(self):
+        lazy_list = SimpleLazyObject(lambda: [1, 2, 3, 4, 5])
+        lazy_set = SimpleLazyObject(lambda: set([1, 2, 3, 4]))
+        self.assertTrue(1 in lazy_list)
+        self.assertTrue(1 in lazy_set)
+        self.assertFalse(6 in lazy_list)
+        self.assertFalse(6 in lazy_set)
+        self.assertEqual(len(lazy_list), 5)
+        self.assertEqual(len(lazy_set), 4)