In Python 3.12 we've made pathlib.PurePath subclassable - see #31691.

A significant wrinkle is that subclasses inherit an __fspath__() method, which can lead to unexpected behaviour when these "pure" paths are passed to filesystem APIs:

import pathlib

class TarPath(pathlib.PurePath):
    ...

readme = TarPath('README.md', tarfile=...)
with open(readme) as f:
   ...

The with open(readme) as f: line should throw a TypeError, but instead it attempts to open a file called README.md in the current working directory (!). This sort of thing makes subclasses that implement purely virtual filesystems difficult to use safely.

Linked PRs

• GH-106037: Disarm os.PathLike foot-shotgun in pathlib.PurePath user subclasses #106043