With this patch 'contextlib.ExitStack.callback(callback, *args, **kwds)' will validate that the passed callback actually accept the passed '*args' and '**kwds'.

P.S. If this patch comes through - I'll create two more issues for 'atexit.register' and 'unittest.TestCase.addCleanup'.  (I'll also look through the stdlib to find if there are some other places where this is needed.)