Reading the documentation for ranges (see https://docs.python.org/3.5/library/stdtypes.html#ranges), the example using a negative index output is inconsistent with the range effective behaviour.

One can read:
"
>>> r[-1]
18
"

while (in my understanding) it should be:
"
>>> r[-1]
8
"

Note the output should be 8 and not 18.

Raphaël