diy2.py

with open(r"C:\Users\Administrator\Desktop\book.txt",'r',encoding='utf-8') as file:
    for data in file:
        if '小兰：12本' in data:
            print('小兰数据在文件中')
with open(r"C:\Users\Administrator\Desktop\book.txt",'w',encoding='utf-8') as file:
    file.write(new_data)   
with open(r"C:\Users\Administrator\Desktop\book.txt",'r',encoding='utf-8') as file:
    a = file.read()
    print(a)