Flatten and Rebuild Nested JSON

from typing import Any


def flatten(data: dict[str, Any], prefix: str = "") -> dict[str, Any]:
    flat: dict[str, Any] = {}
    for key, value in data.items():
        path = f"{prefix}.{key}" if prefix else key
        if isinstance(value, dict) and value:
            flat.update(flatten(value, path))
        else:
            flat[path] = value
    return flat


def unflatten(flat: dict[str, Any]) -> dict[str, Any]:
    root: dict[str, Any] = {}
    for path, value in flat.items():
        parts = path.split(".")
        cursor = root
        for part in parts[:-1]:
            existing = cursor.get(part)
            if existing is None:
                existing = {}
                cursor[part] = existing
            if not isinstance(existing, dict):
                raise ValueError(f"path conflict at {part!r} in {path!r}")
            cursor = existing
        leaf = parts[-1]
        if leaf in cursor:
            raise ValueError(f"duplicate or conflicting path: {path!r}")
        cursor[leaf] = value
    return root


if __name__ == "__main__":
    source = {
        "server": {"host": "localhost", "ports": [8080, 8081], "tls": {"enabled": True}},
        "name": "api",
        "empty": {},
    }
    flat = flatten(source)
    print(flat)
    print(unflatten(flat) == source)
    try:
        unflatten({"a": 1, "a.b": 2})
    except ValueError as error:
        print("rejected:", error)