import logging
import shelve
from pathlib import Path

import pandas as pd


logging.basicConfig(level=logging.INFO)


def cached_download_csv(data_path: Path, url: str, **kwargs) -> pd.DataFrame:
    shelf_filename = data_path.joinpath("shelf.dat").as_posix()
    try:
        s = shelve.open(shelf_filename, writeback=True, flag="c")
    except Exception as exn:
        logging.exception(
            "Unexpected exception, while trying to access the shelf"
        )
        return pd.read_csv(url, **kwargs)
    with s as shelf:
        logging.info(f"Opened or created the shelf file '{shelf_filename}'")
        maybe_cached = shelf.get(url)
        if maybe_cached is None:
            logging.info(f"Downloawding URL '{url}'")
            df = pd.read_csv(url, **kwargs)
            shelf[url] = df
            return df
        else:
            logging.info(f"Re-using cached URL '{url}'")
            assert isinstance(maybe_cached, pd.DataFrame)
            return maybe_cached
    # The context manager 'shelf' ensure set value as written back or return