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

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

import turtlecolors= ['#B03060','#9B30FF','#CD6889','#8470FF','#DB7093','#EE5C42']turtle.speed(10)for i in range(150):    turtle.pencolor(colors[i% 6])    turtle.forward(i)    turtle.left(59)turtle.done()

classmates = ['Micheel','Bob','Tracy']#print(len(classmates))#print(classmates[1])classmates.append('Tom')classmates.insert(1, 'Jack')classmates.pop()classmates.pop(1)classmates[1] = 'Sarah'print(classmates)