文件操作，列表实例NiceHexSpiral

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

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