Python String Slicing Stride Clarification

Question

So I don't really get the deal with the stride parameter in slicing.
For example, "123456"[::-2] produces "642", but why does "123456"[1::-2] produce "2" and "123456"[2::-2] produce "31"?

Answer 1

The easiest way to explain is probably to address your examples:

"123456"[::-2] # This takes the whole string ([::]) # Then it works backward (-) # and it does every other character (2)  "123456"[1::-2] # This is also working backward (-) # every other character (2) # but starting at position 1, which is the number 2.  "123456"[2::-2] # Again, working backward (-) # Every other character (2) # begin at position 2, so you end up with positions 2, and 0, or '31' 

The slicing syntax is [<start>:<end>:step]. If <start> is omitted and the step is negative then it starts at the end of the string.

Answer 2

That's because the syntax is string[start:end:step]

"123456"[::-2] 

Produces "642" because it'll start from the last character. This is because you didn't provide from which position the slice would be executed. So it'll go back by 2 characters from the last one until it reaches the first one.

"123456"[1::-2] 

Produces "2" because you told Python to start from the 2nd character (index 1 from the string) and you told Python to go back 2 steps from that position. Which in this case, Python would obviously return just "2". This is because when Python tried to go back by 2 steps, it already hit the first character in the string with just one step.

"123456"[2::-2] 

You should be able to figure this one out by now. But I'll just explain it anyway. So, you told Python to start from the third character (or index 2) and then go back 2 steps from there until it reaches the first character. So it'll start from "3" and then go back 2 steps, and accidentally the first character - which happens to be "1" - has been reached. So Python will give you "31"