from typing import List
import pandas as pd


def add_regional_features(df: pd.DataFrame, regions: List[str]) -> pd.DataFrame:
    result = df.copy()

    if "region" in result.columns and "real_time_price" in result.columns:
        avg_price_by_region = result.groupby("region")["real_time_price"].mean()

        for region in regions:
            region_avg = avg_price_by_region.get(region, 0)
            result[f"price_diff_{region}"] = result["real_time_price"] - region_avg

    return result


__all__ = ["add_regional_features"]