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Python特殊语法--filter、map、reduce、lambda

一、filter(function, sequence)对sequence中的item依次执行function(item),将执行结果为True的item组成一个List/String/Tuple(取决于sequence的类型)返回:def f(x):     return x % 2 != 0 and x % 3 != 0 print filter(f, range(2, 25)) #[5, 7, 11, 13, 17, 19, 23]def f1(x):     return x != a print filter(f1, "abcdef") #bcdef 二、map(function, sequence)  对sequence中的item依次执行function(item),见执行结果组成一个List返回:def cube(x):     return x*x*x print map(cube, range(1, 11)) #[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]def cube1(x):     return x + x print map(cube1 , "abcde") #[‘aa‘, ‘bb‘, ‘cc‘, ‘dd‘, ‘ee‘] 另外map也支持多个sequence,这就要求function也支持相应数量的参数输入:def add(x, y):     return x+y print map(add, range(8), range(8)) #[0, 2, 4, 6, 8, 10, 12, 14]status_ids = map(lambda x:x.get(status__id), snaps)三、reduce(function, sequence, starting_value)对sequence中的item顺序迭代调用function,如果有starting_value,还可以作为初始值调用,例如可以用来对List求和:def add(x,y):    return x + y print reduce(add, range(1, 11)) 注:1+2+3+4+5+6+7+8+9+10reduce(add, range(1, 11), 20) 注:1+2+3+4+5+6+7+8+9+10+20四、lambda这是Python支持一种有趣的语法,它允许你快速定义单行的最小函数,类似与C语言中的宏,这些叫做lambda的函数,是从LISP借用来的,可以用在任何需要函数的地方: g = lambda x: x * 2 print g(3) #6 print (lambda x: x * 2)(3)  #6五、sortedaa = [   {name:zhangsan, price:20.01, date:2015-01-09T01:00:00Z},   {name:lisi, price:10.01, date:2013-01-09T01:00:00Z},   {name:wangwu, price:0.01, date:2012-01-09T01:00:00Z}]  sorted(aa, key=lambda s:s.amount) #对list进行排序sorted(aa, key=lambda s:s.amount, reverse=True)aa = [<Symbol: Symbol object>, <Symbol: Symbol object>, <Symbol: Symbol object>] sorted(aa, key=lambda s:s["date"]) #对Symbol对象进行排序,date为Symbol属性sorted(aa, key=lambda s:s["date"], reverse=True)六、for特殊用法for i in range(4):    print ise=[x**2 for x in range(4)]print se #[0, 1, 4, 9]se=[x**2 for x in range(10) if not x%2]print se #[0, 4, 16, 36, 64]

 

Python特殊语法--filter、map、reduce、lambda