列表与字符串 的双冒号用法

a  = [1,2,3,4,5,6,7]
b= a[::-3]
print(b)
# [7, 4, 1]
a  = [1,2,3,4,5,6,7]
b= a[::3]
print(b)
# [1, 4, 7]
a  = [1,2,3,4,5,6,7]
b= a[2::3]
print(b)
# [3,6]
a  = [1,2,3,4,5,6,7]
b= a[::-2]
print(b)
# [7, 5, 3, 1]