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[Solved] Subclassing pathlib.PosixPath broken since Python 3.12 (actually its fixed, but workaround broken) (beehaw.org)

submitted 6 months ago* (last edited 6 months ago) by [email protected] to c/programming

8 comments fedilink hide all child comments

Solution was quite easy. Thanks to the user reply here: beehaw.org/comment/3535588 or programming.dev/comment/10034690 (Not sure if the links actually work as expected...)

Hi all. I have a little problem and don't know how to solve. A CLI program in Python is broken since Python 3.12. It was working in Python 3.11. The reason is, that Python 3.12 changed how subclassing of a pathlib.Path works (basically fixed an issue), which now breaks a workaround.

The class in question is:

class File(PosixPath):
    def __new__(cls, *args: Any, **kwargs: Any) -> Any:
        return cls._from_parts(args).expanduser().resolve()  # type: ignore

    def __init__(self, source: str | Path, *args: Any) -> None:
        super().__init__()
        self.__source = Path(source)

    @property
    def source(self) -> Path:
        return self.__source

    @property
    def modified(self) -> Time:
        return Time.fromtimestamp(os.path.getmtime(self))

    @property
    def changed(self) -> Time:
        return Time.fromtimestamp(os.path.getctime(self))

    @property
    def accessed(self) -> Time:
        return Time.fromtimestamp(os.path.getatime(self))

    # Calculate sha512 hash of self file and compare result to the
    # checksum found in given file. Return True if identical.
    def verify_sha512(self, file: File, buffer_size: int = 4096) -> bool:
        compare_hash: str = file.read_text().split(" ")[0]
        self_hash: str = ""
        self_checksum = hashlib.sha512()
        with open(self.as_posix(), "rb") as f:
            for chunk in iter(lambda: f.read(buffer_size), b""):
                self_checksum.update(chunk)
            self_hash = self_checksum.hexdigest()
        return self_hash == compare_hash

and I get this error when running the script:

Traceback (most recent call last):
File "/home/tuncay/.local/bin/geprotondl", line 1415, in 
sys.exit(main())
^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1334, in main
arguments, status = parse_arguments(argv)
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1131, in parse_arguments
default, status = default_install_dir()
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1101, in default_install_dir
steam_root: File = File(path)
^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 97, in __new__
return cls._from_parts(args).expanduser().resolve()  # type: ignore
^^^^^^^^^^^^^^^
AttributeError: type object 'File' has no attribute '_from_parts'. Did you mean: '_load_parts'?

Now replacing `_from_parts` with `_load_parts` does not work either and I get this message in that case:

Traceback (most recent call last):
File "/home/tuncay/.local/bin/geprotondl", line 1415, in 
sys.exit(main())
^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1334, in main
arguments, status = parse_arguments(argv)
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1131, in parse_arguments
default, status = default_install_dir()
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1101, in default_install_dir
steam_root: File = File(path)
^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 97, in __new__
return cls._load_parts(args).expanduser().resolve()  # type: ignore
^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/pathlib.py", line 408, in _load_parts
paths = self._raw_paths
^^^^^^^^^^^^^^^
AttributeError: 'tuple' object has no attribute '_raw_paths'

I have searched the web and don't understand how to fix this. Has anyone an idea what to do?

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[–] slurp 3 points 6 months ago* (last edited 6 months ago) (1 children)

Don't really understand why you're overriding __new__. Surely it'd work better to use:

def __init__(self, source: str | Path, *args: Any) -> None:
        super().__init__(Path(source, *args).expanduser().resolve())

But this removes self.__source and the property. I'm not sure what the advantage of using that is but you could always set that before the line super().__init__(Path(source, *args).expanduser().resolve()).

EDIT: if I've completely misunderstood, please could you explain? I don't really understand what subclassing is trying to achieve here, other than simplifying access to certain os.path functions.

[–] [email protected] 2 points 6 months ago* (last edited 6 months ago) (1 children)

I think in Python 3.11 and prior overriding new was needed in order to subclass it correctly, because it was buggy until 3.12. This was a workaround, if I remember correctly. I already tried without new and setting self__source, but couldn't make it work. I was close!

With your suggestion it works now! I will need to test the app a bit now and if everything looks fine, will update the repository. Did exactly what you said and replaced this part:

# class File(PosixPath):
#     def __new__(cls, *args: Any, **kwargs: Any) -> Any:
#         return cls._load_parts(args).expanduser().resolve()  # type: ignore
#
#     def __init__(self, source: str | Path, *args: Any) -> None:
#         super().__init__()
#         self.__source = Path(source)
#
class File(PosixPath):
    def __init__(self, source: str | Path, *args: Any) -> None:
        self.__source = Path(source)
        super().__init__(Path(source, *args).expanduser().resolve())

Thank you for the help and solution!

Edit: Forgot to answer your question. This could work without subclassing too off course, but doing it this way makes it more clean in my opinion. It's the natural way to me to extend functionality and use the extended version for the purpose its designed to. It's not strictly required and I would have done it otherwise too, if this was problematic and not working.

[–] slurp 2 points 6 months ago

Glad I could help! I wasn't sure if I was missing something in what you were trying to do - I get that in some cases folding in those features can make things a lot easier.