def prefix_keys(d: dict, prefix: str, sep: str = "_") -> dict:
    """
    dict: Returns the input dictionary with prefixed keys.

    Examples:
        >>> a = {"k": "val"}
        >>> prefix_keys(d=a, prefix="demo")
        {"demo_k": "val"}
    """
    keys = [prefix + sep + str(k) for k in d.keys()]
    return {key: val for key, val in zip(keys, d.values())}