def progress_string(i, n):
    """ Helper to build progress string

    Parameters
    ----------
    i : int
        current number
    n : int
        maximal number


    Returns
    -------
    progress_str : str
    """
    width = len(str(n))
    string = "({0:{width}d}/{1:d})".format(i, n, width=width)
    return string