avoid hosts 'pre templated warnings' (#23878)

* avoid hosts 'pre templated warnings'

fixed is_template failing on undefined
fixes #23057

* py3 compat

lib/ansible/template/__init__.py

@@ -503,8 +503,8 @@ class Templar:
         ''' lets us know if data has a template'''
         if isinstance(data, string_types):
             try:
-                new = self.do_template(data)
+                new = self.do_template(data, fail_on_undefined=True)
-            except UndefinedError:
+            except (AnsibleUndefinedVariable, UndefinedError):
                 return True
             except:
                 return False

lib/ansible/vars/__init__.py

@@ -429,8 +429,13 @@ class VariableManager:
             variables['inventory_dir'] = self._inventory.basedir()
             variables['inventory_file'] = self._inventory.src()
             if play:
+                templar = Templar(loader=loader)
+                if templar.is_template(play.hosts):
+                    pattern = 'all'
+                else:
+                    pattern = play.hosts or 'all'
                 # add the list of hosts in the play, as adjusted for limit/filters
-                variables['ansible_play_hosts_all'] = [x.name for x in self._inventory.get_hosts(pattern=play.hosts or 'all', ignore_restrictions=True)]
+                variables['ansible_play_hosts_all'] = [x.name for x in self._inventory.get_hosts(pattern=pattern, ignore_restrictions=True)]
                 variables['ansible_play_hosts'] = [x for x in variables['ansible_play_hosts_all'] if x not in play._removed_hosts]
                 variables['ansible_play_batch'] = [x.name for x in self._inventory.get_hosts() if x.name not in play._removed_hosts]