Python中exit、return、sys.exit()等使用实例和区别

有这样一道题目：  字符串标识符.修改例 6-1 的 idcheck.py 脚本,使之可以检测长度为一的标识符,并且可以识别 Python 关键字,对后一个要求,你可以使用 keyword 模块(特别是 keyword.kelist)来帮你.

我最初的代码是：

代码如下:
#!/usr/bin/env python

import string
import keyword
import sys

#Get all keyword for python
#keyword.kwlist
#['and', 'as', 'assert', 'break', ...]
keyWords = keyword.kwlist

#Get all character for identifier
#string.letters ==> 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
#string.digits  ==> '0123456789'
charForId = string.letters + "_"
numForId = string.digits

idInput = raw_input("Input your words,please!")

if idInput in keyWords:
    print "%s is keyword fot Python!" % idInput
else:
    lenNum = len(idInput)
    if(1 == lenNum):
        if(idInput in charForId and idInput != "_"):
            print "%s is legal identifier for Python!" % idInput
        else:
            #It's just "_"
            print "%s isn't legal identifier for Python!" % idInput

    else:
        if(idInput[0:1] in charForId):
            legalstring = charForId + numForId
            for item in idInput[1:]:
                if (item not in legalstring):
                    print "%s isn't legal identifier for Python!" % idInput
                    sys.exit(0)
            print "%s is legal identifier for Python!2" % idInput
        else:
            print "%s isn't legal identifier for Python!3" % idInput
    

代码完毕后，我测试每一条分支，测试到分支时，必须输入_d4%等包含非法字符的标识符才能进行测试，我最初以为，sys.exit(0)---正常退出脚本，sys.exit(1)非正常退出脚本，但是实际情况是/9sys.exit(1)，仅输出返回码不同）：
代码如下:
  if (item not in legalstring):
      print "%s isn't legal identifier for Python!" % idInput