python学习笔记[1]-凯撒密码

#!/usr/bin/python
#FileName:caesar.py
#Code:UTF-8

import sys

def caesarEnCode(string,tag):
    ChgString = ""

    for s in string:
        if s.islower():
            ChgString += chr((ord(s)-ord('a')+tag)%26+ord('a'))
        elif s.isupper():
            ChgString += chr((ord(s)-ord('A')+tag)%26+ord('A'))
    return ChgString

def caesarDeCode(string,tag):
    ChgString = ""

    for s in string:
        if s.islower():
            ChgString += chr((ord(s)-ord('a')+tag)%26+ord('a'))
        elif s.isupper():
            ChgString += chr((ord(s)-ord('A')+tag)%26+ord('A'))
    return ChgString
    return string

if __name__=='__main__':
    flag=sys.argv[1]
    string=sys.argv[2]
    
    tag = 3

    if flag == 'E':
        restring = caesarEnCode(string,tag)
    elif flag == 'D':
        restring = caesarDeCode(string,tag)

print restring