[Python] subclass pathlib.Path

example.py

"""
example of subclassing pathlib.Path

the current downside is repr() shows a PosixPath or WindowsPath

mypy --strict is satisfied by the call to cast(), and as long as the __new__()
method returns an object of type pathlib.Path, everything else should work,
since the created object will be the same as if it were created with pathlib.Path

mypy will complain, as it does not like the Path type on the last line

this solution is the same as:
https://stackoverflow.com/a/29880095
"""
from pathlib import Path
from typing import Union, cast

PathType = Union[str, Path]


class ExistingPath(Path):
    "a Path that is guaranteed to exist"

    def __new__(cls, value: PathType) -> "ExistingPath":
        # pylint: disable=arguments-differ
        """
        create a new instance of ExistingPath

        overrides Path.__new__() and applies stricter requirements

        this function should return a value of type Path
        """
        if not value:
            raise ValueError("'{value}' must be a path")

        path = Path(value).resolve()

        if not path.exists():
            raise ValueError(f"'{path}' does not exist")

        return cast(ExistingPath, path)


existing_path = ExistingPath("./")
normal_path = Path("./").resolve(strict=True)
TEST_FILE_NAME = "test.txt"
existing_test_file = existing_path / TEST_FILE_NAME
normal_test_file = normal_path / TEST_FILE_NAME
assert existing_test_file == normal_test_file
assert isinstance(existing_test_file, Path)