Here's the opinion of some Google+ userGuido van Rossum:
[...] I was swayed by the elegance of half-open intervals. Especially the invariant that when two slices are adjacent, the first slice's end index is the second slice's start index is just too beautiful to ignore. For example, suppose you split a string into three parts at indices i and j -- the parts would be a[:i], a[i:j], and a[j:].
Google+[Google+ is closed, so link doesn't work anymore. Spoiler alert: that was Guido van RossumHere's an archive link.]