How Python works: What do you know about support for negative indices?
python at rcn.com
Mon Sep 13 22:07:19 CEST 2010
On Sep 10, 2:13 pm, Terry Reedy <tjre... at udel.edu> wrote:
> Reading the third paragraph out of context, one can miss the restriction
> to built-in objects. I had assumed that the conversion using len(), when
> available, happened prior to the __getitem__ call.
Yes, that's a common misconception. It is probably based on
the current wording of the docs and on the PySequence_GetItem()
optimized fast-path for builtin and extension sequences.
>From the users point-of-view, the important thing is that it
(effectively) occurs in the __getitem__() code, that builtin
sequences and extensions support it, and that else where it is
Another way of saying it is: if you are ever writing a __getitem__()
method in Python (even for a subclass of a builtin sequence), then it
is up to you to decide whether to add support for negative indices
More information about the Python-list