How to get the relative path between two absolute paths in Python using pathlib?

Question

In Python 3, I defined two paths using pathlib, say:

from pathlib import Path

origin = Path('middle-earth/gondor/minas-tirith/castle').resolve()
destination = Path('middle-earth/gondor/osgiliath/tower').resolve()

How can I get the relative path that leads from origin to destination? In this example, I'd like a function that returns ../../osgiliath/tower or something equivalent.

Ideally, I'd have a function relative_path that always satisfies

origin.joinpath(
    relative_path(origin, destination)
).resolve() == destination.resolve()

(well, ideally there would be an operator - such that destination == origin / (destination - origin) would always be true)

Note that Path.relative_to is not sufficient in this case, since origin is not a destination's parent. Also, I'm not working with symlinks, so it's safe to assume that there are none if this simplifies the problem.

How can relative_path be implemented?

Answer 1

This is trivially os.path.relpath

import os.path
from pathlib import Path

origin      = Path('middle-earth/gondor/minas-tirith/castle').resolve()
destination = Path('middle-earth/gondor/osgiliath/tower').resolve()

assert os.path.relpath(destination, start=origin) == '..\\..\\osgiliath\\tower'