A friend recently pointed out a quirk in Python with negative indices in lists.
Let’s say you have this code:
#!/usr/bin/python myArray = ["a","b","c","d"] print(myArray[-1])
Intuitively, what do you think this should print? It should probably throw an error, right? Well, you’d be wrong. It actually prints “d”.
Why is this? In Python, specifying a positive number n for a list index means to access the element n items to the right of the beginning of the list. Specifying a negative number actually means the inverse. It means to access the element at the index n places to the left of the end of the list. In essence, count backwards from the end of the sequence.
So, in our example,
myArray[-2] would refer to “c”, and so on.
This Python quirk is documented here.