I updated the Python test suite so now all cases should be covered.

It says greater, not better. It's clear imo.

I forgot to update the tests after changing the actual tests. Thanks for warning me, it's fixed now.

This would involve knowledge about basic Python language features. Please be more familiar with the language before attempting code golf challenges ;-)

In the end, it has to do with how you define the index out of bonds situation.

Yes, and they're well defined in the kata's description last two examples:

helper.page_index(20) # should == -1
helper.page_index(-10) # should == -1 because negative indexes are invalid

In python it's natural to use negative indexing...

... into containers, using the [] access notation (__getitem__). But this is a custom method. There is no obvious correlation between the two, which is to say that your intuitions/assumptions are simply wrong.

So what? The model is the same for all languages, if some language has negative indexing it doesn't mean you should change it for no reason. Not a kata issue.