Modify flatten method to display an empty dict

When a dict is flattened, if a value of an item is empty,
e.g. {"cpu_info": {}}, the item is not displayed.

This patch fixes _flatten method to display such an item including an
empty dict.

Change-Id: I2e4ada0de217c5c4ddea10f3f283c6d8e78083b6
Related-Bug: #1594230

novaclient/tests/unit/test_utils.py

@@ -340,7 +340,8 @@ class FlattenTestCase(test_utils.TestCase):
                     'b4': {'c1': ['l', 'l', ['l']],
                            'c2': 'string'}},
              'a2': ['l'],
-             'a3': ('t',)})
+             'a3': ('t',),
+             'a4': {}})
 
         self.assertEqual({'a1_b1': 1234,
                           'a1_b2': 'string',
@@ -348,7 +349,8 @@ class FlattenTestCase(test_utils.TestCase):
                           'a1_b4_c1': ['l', 'l', ['l']],
                           'a1_b4_c2': 'string',
                           'a2': ['l'],
-                          'a3': ('t',)},
+                          'a3': ('t',),
+                          'a4': {}},
                          squashed)
 
     def test_pretty_choice_dict(self):

novaclient/utils.py

@@ -210,7 +210,7 @@ def _flatten(data, prefix=None):
     if isinstance(data, dict):
         for key, value in six.iteritems(data):
             new_key = '%s_%s' % (prefix, key) if prefix else key
-            if isinstance(value, (dict, list)):
+            if isinstance(value, (dict, list)) and value:
                 for item in _flatten(value, new_key):
                     yield item
             else: