make inventory warnings a bit smarter (#46284)

less annoying for common cases
 add comment for 'tricky' conditional

lib/ansible/cli/__init__.py

@@ -807,7 +807,7 @@ class CLI(with_metaclass(ABCMeta, object)):
         no_hosts = False
         if len(inventory.list_hosts()) == 0:
             # Empty inventory
-            if C.LOCALHOST_WARNING:
+            if C.LOCALHOST_WARNING and pattern not in C.LOCALHOST:
                 display.warning("provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'")
             no_hosts = True
 

lib/ansible/inventory/manager.py

@@ -289,7 +289,9 @@ class InventoryManager(object):
                     if C.INVENTORY_ANY_UNPARSED_IS_FAILED:
                         raise AnsibleError(u'Completely failed to parse inventory source %s' % (source))
         if not parsed:
-            display.warning("Unable to parse %s as an inventory source" % source)
+            if source != '/etc/ansible/hosts' or os.path.exists(source):
+                # only warn if NOT using the default and if using it, only if the file is present
+                display.warning("Unable to parse %s as an inventory source" % source)
 
         # clear up, jic
         self._inventory.current_source = None