pathlib.Path.relative_to() blows up when given a path that's not an ancestor of the path on which relative_to is being called:

>>> pathlib.Path("/usr/bin").relative_to("/etc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pathlib.py", line 822, in relative_to
    .format(str(self), str(formatted)))
ValueError: '/usr/bin' does not start with '/etc'

The equivalent with posixpath.relpath (or ntpath.relpath) works just fine:

>>> posixpath.relpath("/usr/bin", "/etc")
'../usr/bin'

It'd be nice if Path.relative_to supported this type of usage.

Hey! 
A few changes are required to accomplish what you seek, but results could be severe. Path.relative_to() should return an exception if relative path is not part of the original path. However, I have got a solution for that. I am not that familiar with python and I need you to give me some time. I hope you would acknowledge my efforts.
Regards

That's by design. If /etc is a symlink to e.g. /mnt/etc, the proposed result is wrong.

Then perhaps the docstring of relative_to could note this (like relpath does), or a separate method that's explicitly not symlink safe created.

Or better yet, what about a function that does (or at least tries to) give the correct answer in the face of symlinks?

Kindly bumping the issue.
I'd suggest overriding `PurePath.relative_to` in the `Path` class, to something like `PurePath.relative_to(self, other, *, allow_ancestor=False): ...`, which would resolve each ancestor of `self` successively to check whether it is also an ancestor of `other`, so that symlinks would be handled correctly.

This issue is being addressed in #40358 which has open PR for v3.11

messages: + msg206499
title: Allow Path.relative_to() to accept non-ancestor paths -> Re: Allow Path.relative_to() to accept non-ancestor paths