os.path.relpath() assumes '//' root is equivalent to '/'

Bug report

pathlib's Path.relative_to() and os.path.relpath(), the two have different behavior: On Linux os.path.relpath() understands double-slash in path is same as single slash, but Path.relative_to() does not.

For example,

import os
os.path.relpath('//foo/bar', '/foo')                 # results 'bar'

from pathlib import Path
pathlib.PosixPath('//foo/bar').relative_to('/foo')   # raises ValueError

pytest issue https://github.com/pytest-dev/pytest/issues/9894 relates to this.

Your environment

• CPython versions tested on: 3.9.2
• Operating system and architecture: Debian Bullseye

Linked PRs

• gh-103798

Comienza comparando os.path.relpath() y pathlib.PosixPath.relative_to() para el ejemplo de //foo/bar frente a /foo; después, revisa pytest issue #9894 y el PR enlazado gh-103798. Se considera terminado cuando el comportamiento previsto se haya resuelto de forma coherente y el ejemplo esté cubierto por pruebas relevantes.