code/download_checkpoint.py

#!/usr/bin/env python3
"""Download an official Ultralytics checkpoint atomically and verify its digest."""

from __future__ import annotations

import argparse
import hashlib
import shutil
import sys
import urllib.request
from pathlib import Path


OFFICIAL = {
    "yolo26s.pt": {
        "url": "https://github.com/ultralytics/assets/releases/download/v8.4.0/yolo26s.pt",
        "sha256": "646f8bc3fe0a656803d95c294f7852321748cb29d13466a1af8862e2db384a1b",
    }
}


def sha256(path: Path) -> str:
    digest = hashlib.sha256()
    with path.open("rb") as handle:
        for chunk in iter(lambda: handle.read(1024 * 1024), b""):
            digest.update(chunk)
    return digest.hexdigest()


def main() -> None:
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("--model", default="yolo26s.pt", choices=sorted(OFFICIAL))
    parser.add_argument("--output-dir", type=Path, default=Path("models"))
    args = parser.parse_args()

    item = OFFICIAL[args.model]
    args.output_dir.mkdir(parents=True, exist_ok=True)
    destination = args.output_dir / args.model
    expected = item["sha256"]
    if destination.is_file() and sha256(destination) == expected:
        print(f"Already verified: {destination}")
        return

    temporary = destination.with_suffix(destination.suffix + ".part")
    if temporary.exists():
        temporary.unlink()
    request = urllib.request.Request(item["url"], headers={"User-Agent": "cuphead-detector-setup/1.0"})
    print(f"Downloading {item['url']} -> {destination}")
    try:
        with urllib.request.urlopen(request, timeout=120) as response, temporary.open("wb") as output:
            shutil.copyfileobj(response, output, length=1024 * 1024)
        actual = sha256(temporary)
        if actual != expected:
            raise RuntimeError(f"SHA256 mismatch: expected {expected}, got {actual}")
        temporary.replace(destination)
    except Exception:
        temporary.unlink(missing_ok=True)
        raise
    print(f"Verified SHA256 {expected}")
    print(destination.resolve())


if __name__ == "__main__":
    try:
        main()
    except KeyboardInterrupt:
        sys.exit(130)