python 列表推导式、lambda表达式、 ？表达式 sorted

Explicit is better than implicit（明了胜于晦涩），就是说那种方式更清晰就用哪一种方式，不要盲目的都使用表达式。

1. 列表推导式：

例一：

multiples = [i for i in range(30) if i % 3 is 0]
print(multiples)
# Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]

def squared(x):
    return x*x
multiples = [squared(i) for i in range(30) if i % 3 is 0]
print multiples
#  Output: [0, 9, 36, 81, 144, 225, 324, 441, 576, 729]

2. lambda表达式:

add = lambda x, y : x+y
add(1,2)  # 结果为3

 

def get_y(a,b):
     return lambda x:ax+b
y1 = get_y(1,1)
y1(1) # 结果为2

# 需求：将列表中的元素按照绝对值大小进行升序排列
list1 = [3,5,-4,-1,0,-2,-6]
sorted(list1, key=lambda x: abs(x))

当然，也可以如下：

list1 = [3,5,-4,-1,0,-2,-6]
def get_abs(x):
    return abs(x)
sorted(list1,key=get_abs)

只不过这种方式的代码看起来不够Pythonic

3. ？表达式

(1) variable = a if exper else b
(2) variable = (exper and [b] or [c])[0]
(3) variable = exper and b or c

上面三种用法都可以达到目的，类似C语言中 variable = exper ? b : c;即：如果exper表达式的值为true则variable = b，否则，variable = c

例如：

a, b = 1, 2
max = a if a > b else b
max2 = (a > b and [a] or [b])[0]  # list
max3 = a > b and a or b
print max,max2,max3

4. sorted

list1 = [3,5,-4,-1,0,-2,-6]
def get_abs(x):
    return abs(x)
a=sorted(list1,key=get_abs)
print list1,a

 

dica = {"a":2,"c":8,"d":6}
dicb = sorted(dica.items(),key=lambda x:x[0])
print dicb
print sorted(dica.keys())