.pop(-1)中的-1怎么理解?

hunsjie

def print_last_word(words):    """Prints the last word after popping it off."""    word = words.pop(-1)    print word https://learn-python-the-hard-way-zh_cn-translation.readthedocs.org/en/latest/ex25.html
如图所示，为什么输出的结果是删除最后一个单词，-1怎么理解？