Why does s[len(s)-1:-1:-1] not work?

Question

In looking at some python strings and functions, I discovered this strange quirk of python:

s = "hello"
print s[::-1]

which then prints: olleh

However, print s[len(s)-1:-1:-1] doesn't work. My understanding is that it should iterate from the last element s[len(s)-1] to the first element s[0]. However, it just prints an empty string '', which I believe is because in strings of some given length (say, 5), s[4] == s[-1]. But, I don't understand why python decides to use -1 instead of 4, which is the actual len(s).

Furthermore, s[len(s):0:-1] + s[0] works. Why is len(s) a valid index? Does python just convert len(s) to 0 arbitrarily?

P.S. This is in Python 2.7.8, I'm not sure if it also works in 3.x.x

EDIT: Confirmed to be the same in Python 3

Answer 1

The slice notation is this:

s[start:stop:step]

specifically, stop being up to but not including it. Translate this to what you're asking:

s[len(s)-1:-1:-1]

This is, based on the length of 5 for 'hello'

s[4:-1:-1]

or

s[4:4:-1]

which is an empty, or zero-length string.

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I have given a much more in-depth exposition of the slice notation here: Explain Python's slice notation