def _calculate_probability(fitness):
    """ calculate probability.

    Arguments:
    ----------
        fitness {list[float]} -- each indivisuals' fitness

    Returns:
    --------
        prob {list[float]} -- each indivisuals' probability for selecting

    Examples:
    ---------
        >>> fitness = [1, 2, 3, 4]
        >>> prob = _calculate_probability(fitness)
        >>> prob
        [0.1, 0.2, 0.3, 0.4]
    """
    prob_sum = sum(fitness)
    prob = []
    for _fitness in fitness:
        prob.append(_fitness/prob_sum)

    return prob