pathlib relative path finding

#!/usr/bin/env python3

from pathlib import Path
from tempfile import TemporaryDirectory


def relpath(path_to: Path, path_from: Path) -> Path:
    path_to = Path(path_to).resolve()
    path_from = Path(path_from).resolve()
    try:
        for p in (*reversed(path_from.parents), path_from):
            head, tail = p, path_to.relative_to(p)
    except ValueError:  # Stop when the paths diverge.
        pass
    return Path("../" * (len(path_from.parents) - len(head.parents))).joinpath(tail)


a_package_name = "a_package"

with TemporaryDirectory() as td:
    pypi_mirror_root = Path(td)
    web_root = pypi_mirror_root / "web"
    json_path = web_root / "json"
    pkg_json_path = json_path / a_package_name
    pkg_json_pypi_path = web_root / "pypi" / a_package_name / "json"

    json_path.mkdir(parents=True)
    pkg_json_path.touch()
    pkg_json_pypi_path.parent.mkdir(parents=True)

    rel_path = relpath(pkg_json_path, pkg_json_pypi_path)
    pkg_json_pypi_path.symlink_to(rel_path)
    print(f"Symlink made relative from {pkg_json_pypi_path} to {rel_path}")