​向大神们请教个问题：

Question

users = ['admin', 'lily', 'Hack', 'join', 'Mark'] new_users = [] for new_user in users:     new_users.append(new_user.lower()) 为什么这样就可以转写list里的元素为小写  for new_user.lower() in users:    这样就会运行报错，报错-属性错误：“列表”对象没有属性“低” 必须像是上面那种写法吗？list里的元素转小写，有其他的方法吗?

Answer 1

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Answer 2

python虽然是动态语言，但它底层仍然是基于类型系统。for ... in 结构会对可迭代对象进行处理，in 之间的值为迭代对象中的元素，显然 users列表中的每一个元素都是字符串对象。只有字符串对象new_user才有lower方法。而list对象是没有的，具体你可以查看list类与str类for x in [1,2,3]  #相当于x=1,x=2,x=3