"""Util functions for context values."""


def get_context_values(values_from_context, possible_values):
    """Transform values from context into string and check them.

    Args:
        values_from_context (str or None): string with comma separator.
        possible_values (list): Return this if values_from_context is None.

    Returns:
        list: A list with values from context.
    Raises:
        ValueError: If values are inappropriate.
    """
    if not values_from_context:
        return possible_values
    values_list = [
        value.strip() for value in values_from_context.split(',')]
    if not set(values_list).issubset(possible_values):
        raise ValueError(
            'This is incorrect format! Please provide '
            'comma-separated list.\n'
            '{} was provided.\n'
            '{} are available.'.format(values_from_context, possible_values)
        )
    return values_list