Sure, the [::]
is the extended slice operator. It allows you to take substrings. Basically, it works by specifying which elements you want as [begin:end:step], and it works for all sequences. Two neat things about it:
- You can omit one or more of the elements and it does “the right thing”
- Negative numbers for begin, end, and step have meaning
For begin and end, if you give a negative number, it means to count from the end of the sequence. For instance, if I have a list:
l = [1,2,3]
Then l[-1]
is 3, l[-2]
is 2, and l[-3]
is 1.
For the step
argument, a negative number means to work backwards through the sequence. So for a list::
l = [1,2,3,4,5,6,7,8,9,10]
You could write l[::-1]
which basically means to use a step size of -1 while reading through the list. Python will “do the right thing” when filling in the start and stop so it iterates through the list backwards and gives you [10,9,8,7,6,5,4,3,2,1]
.
I’ve given the examples with lists, but strings are just another sequence and work the same way. So a[::-1]
means to build a string by joining the characters you get by walking backwards through the string.