Title: pathlib.Path.resolve() returns path with double slash when resolving a relative path in root directory
Type: behavior Stage: patch review
Components: Library (Lib) Versions: Python 3.8, Python 3.7, Python 3.6
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: QbLearningPython, corona10, killerrex, pitrou, serhiy.storchaka, tuxtimo
Priority: normal Keywords: patch

Created on 2018-05-27 10:58 by QbLearningPython, last changed 2018-07-01 13:45 by corona10.

File name Uploaded Description Edit killerrex, 2018-06-01 18:41 Test script of the pathlib behaviour
Pull Requests
URL Status Linked Edit
PR 7666 open corona10, 2018-06-12 14:32
Messages (4)
msg317790 - (view) Author: (QbLearningPython) Date: 2018-05-27 10:58
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()
# since the interpreter has been launched from root
>>> p = pathlib.Path('spam')
>>> p
# just for checking
>>> p.resolve()
# 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()
>>> p = pathlib.Path('eggs')
>>> p
>>> p.resolve()
# 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()
>>> pathlib.Path('./spam').resolve()
>>> pathlib.Path('./spam/egg').resolve()


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


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

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 ( talks about "UNC shares" but this is not the case (in using a macOs HFS+ filesystem).

PEP 428 ( 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')

    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')

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

However, I also checked the POSIX specification link ( 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/ in the Python 3.6 repository

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
        newpath = path + sep + name

Thank you.
msg318452 - (view) Author: Andres Ayala (killerrex) * Date: 2018-06-01 18:41
Script to reproduce:

import os
import pathlib

# Change to the Root directory

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

Expected output:

Incorrect output
msg319395 - (view) Author: Dong-hee Na (corona10) * Date: 2018-06-12 16:12
For reviewers,

I submitted a patch PR 7666.
Please take a look
msg320837 - (view) Author: Dong-hee Na (corona10) * Date: 2018-07-01 13:45
Please review my pull request.
This PR is pending for 19 days.
Date User Action Args
2018-07-01 13:45:28corona10setmessages: + msg320837
2018-06-12 16:12:25corona10setnosy: + corona10
messages: + msg319395
2018-06-12 14:32:10corona10setkeywords: + patch
stage: test needed -> patch review
pull_requests: + pull_request7281
2018-06-02 10:49:18tuxtimosetnosy: + tuxtimo
2018-06-02 04:35:44serhiy.storchakasetnosy: + pitrou, serhiy.storchaka
2018-06-01 18:41:45killerrexsetfiles: +

messages: + msg318452
2018-06-01 17:06:40terry.reedysetstage: test needed
versions: + Python 3.8, - Python 3.4, Python 3.5
2018-05-30 08:05:14killerrexsetnosy: + killerrex
2018-05-27 10:58:33QbLearningPythoncreate