Repository navigation
User subclasses of pathlib.PurePath are os.PathLike #106037
Description
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.
Activity
pythonGH-106037: Allow subclasses of `pathlib.PurePath` to opt into `…
barneygale commentedon Jun 23, 2023
@brettcannon as co-author of PEP 519, your input here would be greatly appreciated!
Though it's not specified as such, I argue that
os.PathLikehas grown to mean "like a local filesystem path". Functions that acceptos.PathLikeobjects almost always call operating system APIs on their paths. There are notable exceptions (likeos.path.join()) that perform only lexical work, but they're in a pretty small minority. The result is thatopen()etc "work", even on path objects that represent non-local paths such as S3 keys.PurePosixPathandPureWindowsPathinstances and subclasses have always been path-like, so we can't change those.PurePathinstances are impossible to create.PurePathsubclasses have just become possible in 3.12, which is where I propose making this change. What do you think?brettcannon commentedon Jun 23, 2023
If the question is, "can I leave
__fspath__off ofAbstractPath", then I can see that making sense since it is meant for file systems which is not automatic for something which wants to implementpathlib's API.barneygale commentedon Jun 26, 2023
Closing this issue -
PurePathis already path-like and trying to do funky things in subclasses is probably more surprising than PurePath having__fspath__(). The latter may not be surprising at all depending on how you viewPurePath! Alex has pointed me towards a good solution forpathlib.AbstractPath.