from typing import Union
from typing import List


def validate_input(asset_input: Union[str, List]) -> List[str]:
    """Checks if input is list.

    :param asset_input: str, list
        Single asset slug string or list of asset slugs (i.e. BTC).
    :return List of asset slugs.
    :raises ValueError if input is neither a string or list
    """
    if isinstance(asset_input, str):
        return [asset_input]
    elif isinstance(asset_input, list):
        return asset_input
    else:
        raise ValueError("Input should be of type string or list")