Use static vars when computing host vars known to be static (inventory_hostname, inventory_dir etc.).

lib/ansible/template/__init__.py

@@ -268,7 +268,7 @@ class Templar:
         # Clearing cache
         self._cached_result       = {}
 
-    def template(self, variable, convert_bare=False, preserve_trailing_newlines=True, escape_backslashes=True, fail_on_undefined=None, overrides=None, convert_data=True):
+    def template(self, variable, convert_bare=False, preserve_trailing_newlines=True, escape_backslashes=True, fail_on_undefined=None, overrides=None, convert_data=True, static_vars = ['']):
         '''
         Templates (possibly recursively) any given data as input. If convert_bare is
         set to True, the given data will be wrapped as a jinja2 variable ('{{foo}}')
@@ -340,7 +340,10 @@ class Templar:
                 # we don't use iteritems() here to avoid problems if the underlying dict
                 # changes sizes due to the templating, which can happen with hostvars
                 for k in variable.keys():
+                    if k not in static_vars:
                         d[k] = self.template(variable[k], preserve_trailing_newlines=preserve_trailing_newlines, fail_on_undefined=fail_on_undefined, overrides=overrides)
+                    else:
+                        d[k] = variable[k]
                 return d
             else:
                 return variable

lib/ansible/vars/hostvars.py

@@ -28,6 +28,13 @@ from ansible import constants as C
 from ansible.inventory.host import Host
 from ansible.template import Templar
 
+STATIC_VARS = [
+  'inventory_hostname', 'inventory_hostname_short',
+  'inventory_file', 'inventory_dir', 'playbook_dir',
+  'ansible_play_hosts', 'play_hosts', 'groups', 'ungrouped', 'group_names',
+  'ansible_version', 'omit', 'role_names'
+]
+
 try:
     from hashlib import sha1
 except ImportError:
@@ -81,7 +88,7 @@ class HostVars(collections.Mapping):
             result = self._cached_result[sha1_hash]
         else:
             templar = Templar(variables=data, loader=self._loader)
-            result = templar.template(data, fail_on_undefined=False)
+            result = templar.template(data, fail_on_undefined=False, static_vars=STATIC_VARS)
             self._cached_result[sha1_hash] = result
         return result