1.简单的for...[if]...语句
>>> a=[12, 3, 4, 6, 7, 13, 21] >>> newList = [x for x in a] >>> newList [12, 3, 4, 6, 7, 13, 21] >>> newList2 = [x for x in a if x%2==0] >>> newList2 [12, 4, 6]
newList构建了一个与a具有相同元素的List。但是，newList和a是不同的List。执行b=a，b和newList是不同的。
newList2是从a中选取满足x%2==0的元素组成的List。
2.嵌套的for...[if]...语句
嵌套的for...[if]...语句可以从多个List中选择满足if条件的元素组成新的List。下面也举几个例子。
>>>a=[12, 3, 4, 6, 7, 13, 21] >>>b=['a', 'b', 'x'] >>>newList=[(x, y) for x in a for y in b] >>>newList [(12, 'a'), (12, 'b'), (12, 'x'), (3, 'a'), (3, 'b'), (3, 'x'), (4, 'a'), (4, 'b'), (4, 'x'), (6, 'a'), (6, 'b'), (6, 'x'), (7, 'a'), (7, 'b'), (7, 'x'), (13, 'a'), (13, 'b'), (13, 'x'), (21, 'a'), (21, 'b'), (21, 'x')] >>>newList2=[(x, y) for x in a for y in b if x%2==0 and y<'x'] >>>newList2 [(12, 'a'), (12, 'b'), (4, 'a'), (4, 'b'), (6, 'a'), (6, 'b')]
