Fixed #20276 -- Implemented __bool__ for MergeDict

MergeDict evaluates now to False if all contained dicts are empty.
Thanks til for the report and the initial patch.

django/utils/datastructures.py

@@ -14,6 +14,12 @@ class MergeDict(object):
     def __init__(self, *dicts):
         self.dicts = dicts
 
+    def __bool__(self):
+        return any(self.dicts)
+
+    def __nonzero__(self):
+        return type(self).__bool__(self)
+
     def __getitem__(self, key):
         for dict_ in self.dicts:
             try:

tests/utils_tests/test_datastructures.py

@@ -203,6 +203,13 @@ class MergeDictTests(SimpleTestCase):
                            ('key2', ['value2', 'value3']),
                            ('key4', ['value5', 'value6'])])
 
+    def test_bool_casting(self):
+        empty = MergeDict({}, {}, {})
+        not_empty = MergeDict({}, {}, {"key": "value"})
+        self.assertFalse(empty)
+        self.assertTrue(not_empty)
+
+
 class MultiValueDictTests(SimpleTestCase):
 
     def test_multivaluedict(self):