shuffle filter: added optional 'seed' parameter

docsite/rst/playbooks_filters.rst

@@ -185,6 +185,10 @@ To get a random list from an existing  list::
     {{ ['a','b','c']|shuffle }} => ['c','a','b']
     {{ ['a','b','c']|shuffle }} => ['b','c','a']
 
+As of Ansible version 2.3, it's also possible to shuffle a list idempotent. All you need is a seed.::
+
+    {{ ['a','b','c']|shuffle(seed=inventory_hostname) }} => ['b','a','c']
+
 note that when used with a non 'listable' item it is a noop, otherwise it always returns a list
 
 

lib/ansible/plugins/filter/core.py

@@ -217,10 +217,14 @@ def rand(environment, end, start=None, step=None, seed=None):
     else:
         raise errors.AnsibleFilterError('random can only be used on sequences and integers')
 
-def randomize_list(mylist):
+def randomize_list(mylist, seed=None):
     try:
         mylist = list(mylist)
-        shuffle(mylist)
+        if seed:
+            r = Random(seed)
+            r.shuffle(mylist)
+        else:
+            shuffle(mylist)
     except:
         pass
     return mylist