Python 列表推导式 一些小的古怪写法 if else for in join

c = a if a>b else b    //如果a>b返回a，否则返回b>>> a = 1>>> b = 2>>> c = a if a>b else b>>> c2

c = [b,a][a>b]  //如果a>b返回a，否则返回b>>> a = 1>>> b = 2>>> c = [b,a][a>b]>>> c2

c = [ i for i in a if i%2==0 ]  //遍历a，在i为偶数时返回>>> a = [1,2,3]>>> c = [i for i in a if i%2==0]>>> c[2]注意当你用上述方式生成列表，但又不引用而只是作为临时参数时，为减少内存开销，应将sum( [ i for i in a if i%2==0 ] )改成sum( i for i in a if i%2==0 )，

生成集合：c = { i for i in a if i%2==0 }  //遍历a，在i为偶数时返回生成字典：c = { i:i**i for i in a if i%2==0 }  //遍历a，在i为偶数时返回

c = [i+j for i in a for j in b if i%2==0 and j%2==0]    //同时嵌套遍历列表a和b，返回i和j同时为偶数时的和。其中for in a属于外层嵌套，for in b属于内层>>> a = [1,2,3]>>> b = [4,5,6]>>> c = [i+j for i in a for j in b if i%2==0 and j%2==0]>>> c[6, 8]

c = [i if i%2==0 else 1 for i in a]  //遍历a，如果i为偶数直接返回，否则直接返回1。此处if写在for前面要求必须有else项>>> a = [1,2,3]>>> c = [i if i%2==0 else 1 for i in a]>>> c[1, 2, 1]

a,b=b,a   //交换数据>>> a=1>>> b=2>>> a,b=b,a>>> a2>>> b1

''.join['1','2']  //字符list的连接>>> c = ''.join(['1','2'])>>> c'12'