文件的某行的替换

str1 = input('需要修改的部分:')
str2 = input('修改成;')
print(type(str1))
with open('lock.txt','r+',encoding='utf-8') as f1,\
open('123.txt','r+',encoding='utf-8') as f2:
    for i in f1:
        if str1 in i:
            i = i.replace(str1, str2)
        f2.write(i)
需要可以在用w+把新的文件写回去