def flatten_list(list_to_flatten):
    """Flatten a list of lists in one single list.

    Parameters
    ----------
    list_to_flatten : List of list. To flatten.

    Returns
    -------
    flatted_list : List. Single list.

    """
    flatted_list = [item for sublist in list_to_flatten for item in sublist]
    return flatted_list