Update HACKING.rst per recent changes

Added "is not" usage with examples
* https://review.openstack.org/#/c/20865/

Fixed "not in" usage description
* https://review.openstack.org/#/c/20875/

Fixed some "not X in Y" corner cases.

Change-Id: I7534ef73e6fd525fd8f4bee594a4b37524699c08

HACKING.rst

@@ -28,16 +28,23 @@ General
 
     mylist = Foo().list() # OKAY, does not shadow built-in
 
+- Use the "is not" operator when testing for unequal identities. Example::
 
-- Use the "not in" operator for collection membership evaluation. Example::
+    if not X is Y:  # BAD, intended behavior is ambiguous
+        pass
 
-    if not X in Y:  # BAD, hard to understand
+    if X is not Y:  # OKAY, intuitive
+        pass
+
+- Use the "not in" operator for evaluating membership in a collection. Example::
+
+    if not X in Y:  # BAD, intended behavior is ambiguous
         pass
 
     if X not in Y:  # OKAY, intuitive
         pass
 
-    if not (X in Y or X is Z):  # OKAY, still better than all those 'not's
+    if not (X in Y or X in Z):  # OKAY, still better than all those 'not's
         pass
 
 

nova/scheduler/filters/json_filter.py

@@ -32,7 +32,7 @@ class JsonFilter(filters.BaseHostFilter):
         if len(args) < 2:
             return False
         if op is operator.contains:
-            bad = not args[0] in args[1:]
+            bad = args[0] not in args[1:]
         else:
             bad = [arg for arg in args[1:]
                     if not op(args[0], arg)]