human
Solve the following python programming problem given the following code:
def test_wait_for_code_changes(self) : 
    result = [] 
    (r, w) = os.pipe () 
    pid = os.fork () 
    if pid : 
        os.close (w) 
        sleep (1) 
        self.f.write ('something') 
        self.f.flush () 
        sleep (1) 
        r = os.fdopen (r) 
        self.assertEqual ('Changes in file "temp_dir/tempfile"', r.read ()) 
        os.kill (pid, signal.SIGKILL) 
        os.waitpid (pid, 0) 
else : 
        os.close (r) 
        w = os.fdopen (w, 'w') 
        def writeback_and_stop(message, filename, * args, **kwargs) : 
            w.write ((message % filename)) 
            w.close () 
Logger.info = Mock (side_effect = <MASK>) 
        app.wait_for_code_change (extra_files = ('temp_dir/nonexistent', 'temp_dir/tempfile')) 
        os._exit (0) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: w, r, pid, result, self
assistant
#Here is a proper replacement for <MASK>.
def writeback_and_stop(