文件操作

import turtle
colors=['yellow','red','black','blue','purple','orange']
turtle.speed(10)
for i in range(100):
    turtle.pencolor(colors[i%6])
    turtle.forward(i)
    turtle.left(59)

turtle.done()

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

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


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