python [ : :-1] how this works

str = 'abc'

print(str[::-1])

--> 'cba'

arr = ['a', 'b', 'c']

print(arr[::-1])

--> ['c', 'b', 'a']

 

used for string reverse 
eg: 
a="wills"
a[::-1]
will give:- "slliw"

即，用于数组的倒序输出。