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：

#!/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）：

  if (item not in legalstring):

      print "%s isn't legal identifier for Python!" % idInput

     sys.exit(0)
Input your words,please!_d4%

_d4% isn't legal identifier for Python!
Traceback (most recent call last):

  File "E:/python/idcheck.py", line 37, in <module>

    sys.exit(0)

SystemExit: 0

>>>

由此可见，这样做没有达到我预期如下输出的效果，那么，问题在哪里呢？在于sys.exit()始终会抛出一个SystemExit异常。

Input your words,please!_d4%

_d4% isn't legal identifier for Python!

#!/usr/bin/env python
import string

import keyword

import sys

import traceback
try:

    #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()

                print "%s is legal identifier for Python!2" % idInput

            else:

                print "%s isn't legal identifier for Python!3" % idInput
except SystemExit:

    pass

except:

    traceback.print_exc()

上面的代码获取sys.exit()抛出的SystemExit异常。

return：在定义函数时从函数中返回一个函数的返回值，终止函数的执行。

exit:下面的代码中，如果把sys.exit()替换成exit，则exit仅仅跳出离它最近的for循环， print "%s is legal identifier for Python!2" % idInput语句会被输出，这里，exit的作用类似于break. 但实际上break和exit作用并不同

                for item in idInput[1:]:

                    if (item not in legalstring):

                        print "%s isn't legal identifier for Python!" % idInput

                        sys.exit()

                print "%s is legal identifier for Python!2" % idInput