pathlib relative_to() can give confusing error message

[cjerdonek]

BPO	23082
Nosy	@pitrou, @cjerdonek, @zooba, @miss-islington, @tirkarthi, @idomic, @rotuna
PRs	bpo-23082: Better error message for PurePath.relative_to() from pathlib #19611 [3.9] bpo-23082: Better error message for PurePath.relative_to() from pathlib (GH-19611) #20395

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/pitrou'
closed_at = <Date 2020-05-25.19:43:02.507>
created_at = <Date 2014-12-18.13:37:19.235>
labels = ['easy', 'type-bug', 'library', '3.9', '3.10']
title = 'pathlib relative_to() can give confusing error message'
updated_at = <Date 2020-05-25.20:01:26.106>
user = 'https://github.com/cjerdonek'

bugs.python.org fields:

activity = <Date 2020-05-25.20:01:26.106>
actor = 'miss-islington'
assignee = 'pitrou'
closed = True
closed_date = <Date 2020-05-25.19:43:02.507>
closer = 'steve.dower'
components = ['Library (Lib)']
creation = <Date 2014-12-18.13:37:19.235>
creator = 'chris.jerdonek'
dependencies = []
files = []
hgrepos = []
issue_num = 23082
keywords = ['patch', 'easy', 'newcomer friendly']
message_count = 7.0
messages = ['232876', '232877', '366433', '366568', '369838', '369914', '369919']
nosy_count = 8.0
nosy_names = ['pitrou', 'Arfrever', 'chris.jerdonek', 'steve.dower', 'miss-islington', 'xtreak', 'Ido Michael', 'rotuna']
pr_nums = ['19611', '20395']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23082'
versions = ['Python 3.9', 'Python 3.10']

[cjerdonek]

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.

[cjerdonek]

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.

[zooba]

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.

[rotuna]

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'

[idomic]

Hey looks like this PR is good to go?

[zooba]

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

[miss-islington]

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