bpo-21041: Add negative indexing to pathlib path parents

[ypankovych]

As I can see, pathlib path parents don't support negative indexing:

>>> import pathlib
>>> path = pathlib.PosixPath("some/very/long/path/here")
>>> path.parents[-1]
...
    raise IndexError(idx)
IndexError: -1

That's kinda weird for python. I mean, in regular list/etc if I need the last element, I'd normally do list[-1], but here to get the last parent, I need to actually know how many parents do I have.

So now, I can do something like this:

>>> parents_count = len(path.parents) - 1
>>> path.parents[parents_count]
PosixPath('.')

So that's how I can get the last parent. But is it pythonic? No.

So, I decided to fix this, and now we can do negative indexing:

>>> path.parents[-1] == path.parents[parents_count]
True
>>> path.parents[-2] == path.parents[parents_count - 1]
True

https://bugs.python.org/issue21041

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[thejcannon]

Minor: The title says "negative slicing", however what this change is fixing is negative indexing.

[ypankovych]

Minor: The title says "negative slicing", however what this change is fixing is negative indexing.

Thanks, fixed

[remilapeyre]

Hi @ypankovych, thanks for writing this PR. It's not sure this change will be accepted but it will require a NEWS entry, you can add one by using https://blurb-it.herokuapp.com/

[ypankovych]

@remilapeyre thanks, done.

[remilapeyre]

Whether or not this is the wanted behaviour still needs to be discussed on the bpo issue, but the implementation looks correct to me. Thanks @ypankovych !

[ypankovych]

@remilapeyre thanks for your review.

[pganssle]

@ypankovych Since we're changing the semantics of this, could you please add a .. versionchanged:: 3.10 annotation to the pathlib documentation, and add a "What's New" entry for Python 3.10?

Other than that, this looks good to me. Given the lack of explicit objections (Chesterton's fence aside) and the fact that this has been stalled on no one taking decisive action for the past 6 years or so, I'm inclined to go ahead and merge this once it has a What's New entry.

@pitrou As the original author of pathlib, do you have any objections to that course of action?

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[pitrou]

I have no objection to this :-) I'm not sure why I didn't add support for negative indexing, but it's probably laziness.

[pganssle]

@ypankovych I have merged #11165, which introduced some merge conflicts with this PR. Can you re-base against the master branch and resolve the conflicts?

You'll also want to modify the .. versionchanged:: 3.10 directive in the documentation to say something like "The parents sequence now supports a :term:slice or negative index values."

[ypankovych]

I have made the requested changes; please review again

[bedevere-bot]

Thanks for making the requested changes!

@pganssle: please review the changes made to this pull request.