diy.py

nd="";
# 写入同学们的捐赠明细：'小兰：12本'、'小丽：11本'、'李文：9本'、'张伟：16本'
with open(r'C:\Users\MKHG1\Desktop\test.txt','a',encoding='utf-8') as file1:
    file1.write('小兰：12本\n')
    file1.write('小丽：11本\n')
    file1.write('李文：9本\n')
    file1.write('张伟：16本\n')
with open(r'C:\Users\MKHG1\Desktop\test.txt','r',encoding='utf-8') as file1:
    for i in file1:
        if "小兰：12本" in i:
            i=i.replace("小兰：12本","小兰：20本")
        nd+=i;
with open(r'C:\Users\MKHG1\Desktop\test.txt','w',encoding='utf-8') as file1: 
    file1.write(nd)
with open(r'C:\Users\MKHG1\Desktop\test.txt','r',encoding='utf-8') as file1:
    a=file1.read()
    print(a)