def del_none_col(df,threshold = 0.5):
    """delete those columns whose none values number is bigger than threshold."""
    l = []
    for each in df:
        if df[each].isnull().sum()/len(df[each]) > threshold:
            l.append(each)
    df.drop(l,axis = 1,inplace = True)
    if len(l) > 0:
        print(f"Deleted {len(l)} features below:")
        return l
    else:
        print('No qualified feature to delete')