This function is for setting variables on hosts. So those two parts are always required.

Don't have to check for the existence of those in the function.  If they
aren't set to iterables then it should be up to the calling code to
handle it.

lib/ansible/plugins/inventory/__init__.py

@@ -74,11 +74,8 @@ class BaseInventoryPlugin(object):
         pass
 
     def populate_host_vars(self, hosts, variables, group=None, port=None):
-
-        if hosts:
         for host in hosts:
             self.inventory.add_host(host, group=group, port=port)
-                if variables:
             for k in variables:
                 self.inventory.set_variable(host, k, variables[k])