I was asked on Twitter why Python uses 0-based indexing, with a link to a new (fascinating) post on the subject (http://exple.tive.org/blarg/2013/10/22/citation-needed/). I recall thinking about it a lot; ABC, one of Python's predecessors, used 1-based indexing, while C, the other big influence, used 0-based. My first few programming languages (Algol, Fortran, Pascal) used 1-based or variable-based. I think that one of the issues that helped me decide was slice notation.
Let's first look at use cases. Probably the
most common use cases for slicing are "get the first n items" and "get
the next n items starting at i" (the first is a special case of that for
i == the first index). It would be nice if both of these could be
expressed as without awkward +1 or -1 compensations.
0-based indexing, half-open intervals, and suitable defaults (as Python
ended up having), they are beautiful: a[:n] and a[i:i+n]; the former is
long for a[0:n].
Using 1-based indexing, if you want a[:n] to
mean the first n elements, you either have to use closed intervals or
you can use a slice notation that uses start and length as the slice
parameters. Using half-open intervals just isn't very elegant when
combined with 1-based indexing. Using closed intervals, you'd have to
write a[i:i+n-1] for the n items starting at i. So perhaps using the
slice length would be more elegant with 1-based indexing? Then you could
write a[i:n]. And this is in fact what ABC did -- it used a different
notation so you could write a@i|n.(See http://homepages.cwi.nl/~steven/abc/qr.html#EXPRESSIONS.)
how does the index:length convention work out for other use cases? TBH
this is where my memory gets fuzzy, but I think 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:].
So that's why Python uses 0-based indexing.