Serhiy Storchaka added the comment: Sorry for poor words. I was not going to blame anybody. But changing interface always is dangerous. It makes third-party classes that implemented the interface no longer valid substitutions. I think two things are worth to be done: 1) Explicitly document (including the docstring) that the support of stop and start arguments is optional. Not all sequences implements it. 3.5+. 2) Add the support of stop and start arguments to range() in 3.7. And would be nice to provide a way for testing if the sequence supports extended index(). If this is possible. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue28197> _______________________________________