1 files changed, 18 insertions, 0 deletions
diff --git a/nova/utils.py b/nova/utils.py
index 9cfb6d06d..adb1cf45c 100644
--- a/nova/utils.py
+++ b/nova/utils.py
@@ -861,6 +861,24 @@ def subset_dict(dict_, keys):
     return subset
 
 
+def diff_dict(orig, new):
+    """
+    Return a dict describing how to change orig to new.  The keys
+    correspond to values that have changed; the value will be a list
+    of one or two elements.  The first element of the list will be
+    either '+' or '-', indicating whether the key was updated or
+    deleted; if the key was updated, the list will contain a second
+    element, giving the updated value.
+    """
+    # Figure out what keys went away
+    result = dict((k, ['-']) for k in set(orig.keys()) - set(new.keys()))
+    # Compute the updates
+    for key, value in new.items():
+        if key not in orig or value != orig[key]:
+            result[key] = ['+', value]
+    return result
+
+
 def check_isinstance(obj, cls):
     """Checks that obj is of type cls, and lets PyLint infer types."""
     if isinstance(obj, cls):