I have recently found a weird behaviour while trying to resolve a relative path located on the root directory on a macOs.

I tried to resolve a Path('spam') and the interpreter answered PosixPath('//spam') —double slash for root— instead of (my) expected PosixPath('/spam').

I think that this is a bug.

I ran the interpreter from root directory (cd /; python). Once running the interpreter, this is what I did:

>>> import pathlib
>>> pathlib.Path.cwd()
PosixPath('/')
# since the interpreter has been launched from root
>>> p = pathlib.Path('spam')
>>> p
PosixPath('spam')
# just for checking
>>> p.resolve()
PosixPath('//spam')
# beware of double slash instead of single slash


I also checked the behaviour of Path.resolve() in a non-root directory (in my case launching the interpreter from /Applications).

>>> import pathlib
>>> pathlib.Path.cwd()
PosixPath('/Applications')
>>> p = pathlib.Path('eggs')
>>> p
PosixPath('eggs')
>>> p.resolve()
PosixPath('/Applications/eggs')
# just one slash as root in this case (as should be)

So it seems that double slashes just appear while resolving relative paths in the root directory.

More examples are:
	
>>> pathlib.Path('spam/egg').resolve()
PosixPath('//spam/egg')
>>> pathlib.Path('./spam').resolve()
PosixPath('//spam')
>>> pathlib.Path('./spam/egg').resolve()
PosixPath('//spam/egg')

but

>>> pathlib.Path('').resolve()
PosixPath('/')
>>> pathlib.Path('.').resolve()
PosixPath('/')

Intriguingly,

>>> pathlib.Path('spam').resolve().resolve()
PosixPath('/spam')
# 'spam'.resolve = '//spam'
# '//spam'.resolve = '/spam'!!!
>>> pathlib.Path('//spam').resolve()
PosixPath('/spam')

I have found the same behaviour in several Python versions:

Python 3.6.5 (default, May 15 2018, 08:20:57)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.1)] on darwin

Python 3.4.8 (default, Mar 29 2018, 16:18:25)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)] on darwin

Python 3.5.5 (default, Mar 29 2018, 16:22:58)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)] on darwin

Python 3.7.0b4 (default, May 4 2018, 22:01:49)
[Clang 9.1.0 (clang-902.0.39.1)] on darwin


All running on: macOs High Sierra 10.13.4 (17E202)


There is also confirmation of same issue on Ubuntu 16.04 (Python 3.5.2) and Opensuse tumbleweed (Python 3.6.5)


I have searched for some information on this issue but I did not found anything useful.

Python docs (https://docs.python.org/3/library/pathlib.html) talks about "UNC shares" but this is not the case (in using a macOs HFS+ filesystem).

PEP 428 (https://www.python.org/dev/peps/pep-0428/) says:


    Multiple leading slashes are treated differently depending on the path flavour. They are always retained on Windows paths (because of the UNC notation):

    >>> PureWindowsPath('//some/path')
    PureWindowsPath('//some/path/')

    On POSIX, they are collapsed except if there are exactly two leading slashes, which is a special case in the POSIX specification on pathname resolution [8] (this is also necessary for Cygwin compatibility):

    >>> PurePosixPath('///some/path')
    PurePosixPath('/some/path')
    >>> PurePosixPath('//some/path')
    PurePosixPath('//some/path')


I do not think that this is related to the aforementioned issue.

However, I also checked the POSIX specification link (http://pubs.opengroup.org/onlinepubs/009...#tag_04_11) and found:

    A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.


I do not really think that this can cause a double slashes while resolving a relative path on macOs.


So, I think that this issue could be a real bug in pathlib.Path.resolve() method. Specifically on POSIX flavour.

A user of Python Forum (killerrex) and I have traced the bugs to Lib/pathlib.py:319 in the Python 3.6 repository https://github.com/python/cpython/blob/3...pathlib.py.

Specifically, in line 319:
	
    newpath = path + sep + name

For pathlib.Path('spam').resolve() in the root directory, newpath is '//spam' since:

    path is '/'
    sep is '/'
    name is 'spam'

killerrex has suggested two solutions:

1) from line 345 

    base = '' if path.is_absolute() else os.getcwd()
    if base == sep:
        base = ''
    return _resolve(base, str(path)) or sep

2) from line 319:

    if path.endswith(sep):
        newpath = path + name
    else:
        newpath = path + sep + name


Thank you.

Script to reproduce:

import os
import pathlib

# Change to the Root directory
os.chdir('/')

# Create a relative path object.
p = pathlib.Path('spam')
print(p.resolve())


Expected output:
/span

Incorrect output
//span

For reviewers,

I submitted a patch PR 7666.
Please take a look

Please review my pull request.
This PR is pending for 19 days.

This issue cropped up recently in the black project: https://github.com/psf/black/issues/1631

It appears to me that PR 7666 was closed without being merged.

I created https://github.com/python/cpython/pull/21971 before I had found this issue.

New changeset 94ad6c674f7687ef22853cb8d42b440d6b42ddc8 by Łukasz Langa (Dong-hee Na) in branch 'master':
bpo-33660: Fix PosixPath to resolve a relative path on root
https://github.com/python/cpython/commit/94ad6c674f7687ef22853cb8d42b440d6b42ddc8

New changeset 7475aa2c590e33a47f5e79e4079bca0645e93f2f by Miss Islington (bot) in branch '3.8':
bpo-33660: Fix PosixPath to resolve a relative path on root (GH-21975)
https://github.com/python/cpython/commit/7475aa2c590e33a47f5e79e4079bca0645e93f2f

New changeset 211e4c6e9c1ab60bb2577dda6587fbec79f679b2 by Miss Islington (bot) in branch '3.9':
bpo-33660: Fix PosixPath to resolve a relative path on root (#21974)
https://github.com/python/cpython/commit/211e4c6e9c1ab60bb2577dda6587fbec79f679b2

Sorry it took so long, this will get released in Python 3.8.6 and newer.

Thanks Łukasz for finanlizing this work :)