pathlib's relative_to(other) can give a confusing message when "other" is os.curdir.

For example--

    Python 3.4.2 (default, Nov 12 2014, 18:23:59) 
    [GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.54)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os
    >>> from pathlib import Path
    >>> Path("/foo").relative_to(os.curdir)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/pathlib.py", line 806, in relative_to
        .format(str(self), str(formatted)))
    ValueError: '/foo' does not start with ''

I guess the error here is that the path must be relative when "other" is relative.

By the way, here is another (less) confusing error message:

    >>> Path("foo").relative_to("fo")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/pathlib.py", line 806, in relative_to
        .format(str(self), str(formatted)))
    ValueError: 'foo' does not start with 'fo'

Without knowing that "foo" is a path, the message seems wrong. If it said something like "Path 'foo' does not start with part 'fo'", it would be clearer.

I agree it's worth improving the error message (and probably the docs too).

It's never made clear that relative_to only looks deeper (won't ever generate leading ".." parts) - the closest hint in the docs is that os.path.relpath is different (and that isn't even in the relative_to() section).

Tagging this easy/newcomer friendly. If you'd like to work on it, just post here - I'm happy to help get it merged.

I'll work on this.

I tried to come up with a single error message but having two different error messages seems like a better idea to me. One for when the path isn't a subpath and one for when absolute and relative paths are mixed. Basically to explain this:

>>> Path("/Users/rotuna/Documents/cpython/Libs").relative_to(Path("./Libs"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/rotuna/Documents/cpython/Lib/pathlib.py", line 907, in relative_to
    raise ValueError("{!r} is not a subpath of{!r}. NOTE: If this is not true, use absolute paths"
ValueError: '/Users/rotuna/Documents/cpython/Libs' does not start with 'Libs'

Hey looks like this PR is good to go?

New changeset 4483253 by Rotuna in branch 'master':
bpo-23082: Better error message for PurePath.relative_to() from pathlib (GH-19611)
4483253

New changeset 318a18e by Miss Islington (bot) in branch '3.9':
bpo-23082: Better error message for PurePath.relative_to() from pathlib (GH-19611)
318a18e