What does [::-1} do?

The syntax [::-1] is used in Python to create a reversed copy of a sequence, such as a string, list, or tuple. It is called extended slicing and is a concise way to reverse the order of elements in the sequence.

Here’s how it works:

• When used with a positive step value (1), like [::-1], it returns a new sequence with elements in reverse order.

For example, consider a string hello = "Hello, World!". Using hello[::-1] will give you a new string '!dlroW ,olleH', which is the reversed version of the original string.

• If you have a list or tuple, the behavior is the same. For instance, given a list numbers = [1, 2, 3, 4, 5], numbers[::-1] will produce [5, 4, 3, 2, 1], which is a reversed copy of the original list.
• You can also use [start:end:step] to extract a subsequence with a reversed order. For instance, hello[7:12:-1] will return 'W ,ol', which is a subsequence consisting of characters from index 7 to 12 (exclusive) in reverse order.

Overall, [::-1] is a useful syntax in Python for quickly reversing a sequence without modifying the original object.