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

先比较 //foo/bar 与 /foo 这个示例下的 os.path.relpath() 和 pathlib.PosixPath.relative_to()，然后查看 pytest issue #9894 以及关联的 PR gh-103798。当预期行为得到一致解决，并且该示例由相关测试覆盖时，即视为完成。