os.path.commonpath() not so common

[filipp]

BPO	39250
Nosy	@zware, @Filipp

^{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 = None
closed_at = <Date 2020-01-07.20:45:15.710>
created_at = <Date 2020-01-07.20:27:13.303>
labels = ['3.8', '3.7', 'library']
title = 'os.path.commonpath() not so common'
updated_at = <Date 2020-01-07.20:45:15.709>
user = 'https://github.com/filipp'

bugs.python.org fields:

activity = <Date 2020-01-07.20:45:15.709>
actor = 'zach.ware'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-07.20:45:15.710>
closer = 'zach.ware'
components = ['Library (Lib)']
creation = <Date 2020-01-07.20:27:13.303>
creator = 'filipp'
dependencies = []
files = []
hgrepos = []
issue_num = 39250
keywords = []
message_count = 4.0
messages = ['359535', '359537', '359541', '359542']
nosy_count = 2.0
nosy_names = ['zach.ware', 'filipp']
pr_nums = []
priority = 'normal'
resolution = 'works for me'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39250'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[filipp]

The documentation describes os.path.commonpath() as:

"Return the longest common sub-path of each pathname in the sequence paths. Raise ValueError if paths contain both absolute and relative pathnames, the paths are on the different drives or if paths is empty. Unlike commonprefix(), this returns a valid path."

However, in practice the function seems to always return the *shortest* common path. Steps to reproduce:

import os.path
paths = ['/var', '/var/log', '/var/log/nginx']
os.path.commonpath(paths)

Expected results:
'/var/log'

Actual results:
'/var'

I've tried this with Python 3.5, 3.6, 3.7 and 3.8.1 on both MacOS and Debian/Linux and the results are consistent.

[zware]

I'm not seeing how /var has /var/log in common with /var/log and /var/log/nginx.

[filipp]

Hi Zach!

You're absolutely right. Forgive my ignorance. Too much staring at the trees and not seeing the forest (or vice versa).

commonpath() works as advertised. I'm just looking for something different.

Sorry for the false alarm. You can close this issue

[zware]

No problem :)