List Comprehensions 不知道具体的翻译表达,也懒得去弄清楚国内行业中是怎么表达的.暂且就叫它为’列表推导式’.
总之感觉这个语法糖还是很优雅的.列表推导式的作用就是创建一个List,方法比较简洁.在一般的程序中,会把满足一定条件的结果放到预先定义好的list对象中.
squares = [] >>> for x in range(10): ... squares.append(x**2) ... >>> squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
针对上的运算,我们可以通过下面的方式得到相同的结果:
squares = [x**2 for x in range(10)]
当然上面的表达式同样可以通过lambda表达实现:
squares = map(lambda x: x**2, range(10)),
但是它相比较来说更加容易让人理解.
同样列表推导式还可以包含一些括号括起来的运算逻辑.而且还可以有很多的for或者if判断.
[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y] [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
上面的表达式等同于:
combs = [] >>> for x in [1,2,3]: ... for y in [3,1,4]: ... if x != y: ... combs.append((x, y)) ... >>> combs [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
如果表达式是一个元组,就需要把它括起来:
vec = [-4, -2, 0, 2, 4] # create a new list with the values doubled [x2 for x in vec] # filter the list to exclude negative numbers [x for x in vec if x >= 0] # apply a function to all the elements [abs(x) for x in vec] # call a method on each element freshfruit = [' banana', ' loganberry ', 'passion fruit '] [weapon.strip() for weapon in freshfruit] # create a list of 2-tuples like (number, square) [(x, x2) for x in range(6)] # the tuple must be parenthesized, otherwise an error is raised [x, x2 for x in range(6)] File "
相关链接:
http://docs.python.org/tutorial/datastructures.html#list-comprehensions
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