python中的列表推导浅析

list1 = [1,2,4,5,12]
list2 = []
for item in list1:
    list2.append(item*2)
print list2

list1 = [1,2,4,5,12]
list2 = [item*2 for item in list1 ]
print list2

list1 = [1,2,4,5,12]
list2 = [item for item in list1 if item < 10 ]
print list2

list1 = [1,2,3]
list2 = [4,5,6]
list3 = [(item1,item2) for item1 in list1 for item2 in list2 ]
print list3

matrix1 = [
          [1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12]
          ]
matrix2 = [[row[i] for row in matrix1] for i in range(4)]
for row in matrix2:
    print row