python中这样的写法叫什么，有什么规则？
[x for x in os.listdir('.') if os.path.isdir(x)]列表解析(List Comprehension)
[expression for variable in iterable]可选条件子句
[expression for variable in iterable if condition]可以多重循环
[expression for variable1 in iterable1 for variable2 in iterable2]