diy.py

#with open(r"C:\Users\mk\Desktop\book.txt",'w',encoding='utf-8') as file:
#    file.write('小兰:12本\n')
#    file.write('小丽:11本\n')
#    file.write('李文:9本\n')
with open(r"C:\Users\mk\Desktop\book.txt",'r',encoding = 'utf-8') as file:#打开文件
    for date in file: #遍历file date为储存数据的量
        if '小兰:12本' in date: #判断'小兰:12本'是否date里
            print('数据在文件里') #输出