def unique_lst(lst,
               size=None):
    """
        Get the unique values inside a list
        ----
        Parameters:
            lst (list) : list of values
            size (int) : number of unique elements the list must contain.
                         By default it is set to None, then all the element
                         of the list will be checked.
        ----
        Returns:
            unique_lst (list) : list of the unique values in lst
    """

    assert (len(lst) > 0)

    unique_lst = []
    el_num = 0

    for el in lst:

        if el not in unique_lst:
            unique_lst.append(el)
            el_num += 1

        if size != None and el_num == size:  # size limit reached
            break

    assert (len(unique_lst) > 0)

    return unique_lst