检查子进程是否被用户终止

问：

# Create a new terminal window and run the ffmpeg command in it
cmd = ' '.join(ffmpeg.compile(output_stream, overwrite_output=True))
compressing = subprocess.Popen('start cmd /k {}'.format(cmd), shell=True)

while compressing.poll() is None: #check if the process is still running
    print('Processing video...')
    break
else:
    if compressing.returncode == 0: #check if the process was canceled by the user
       print('Video processing canceled by user!')
    else: #if the process was completed successfully
        print('Video processing complete!')

我想检查进程（终端）是否被用户关闭，它应该打印该句子，但是即使用户杀死了子进程终端，它也总是卡住了"Processing Video"