文件操作

fr=open('12.txt',mode='r',encoding='utf-8')
plaincode=fr.read()
print('明文:'+plaincode)
print('密文:',end='')
for c in plaincode:
    print (chr(ord(c)+3),end='')

fr = open('12333.txt',mode='r',encoding='utf-8')
plaincode = fr.read()
fr.close


fw = open('c_12333.txt',mode='a',encoding='utf-8')
for c in plaincode:
    fw.write(chr(ord(c)+3))
fw.close

fr = open('12333.txt',mode='r',encoding='utf-8')
plaincode = fr.read()
fr.close


fw = open('c_12333.txt',mode='a',encoding='utf-8')
for c in plaincode:
    fw.write(chr(ord(c)-3))
fw.close