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Python中exit、return、sys.exit()等使用實例和區(qū)別

更新時間：2015年05月28日 17:47:12 投稿：junjie

這篇文章主要介紹了Python中exit、return、sys.exit()等使用實例和區(qū)別,本文是一個實際項目中的總結(jié),需要的朋友可以參考下

有這樣一道題目：字符串標識符.修改例 6-1 的 idcheck.py 腳本,使之可以檢測長度為一的標識符,并且可以識別 Python 關(guān)鍵字,對后一個要求,你可以使用 keyword 模塊(特別是 keyword.kelist)來幫你.

我最初的代碼是：

復(fù)制代碼代碼如下:

#!/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

代碼完畢后，我測試每一條分支，測試到分支時，必須輸入_d4%等包含非法字符的標識符才能進行測試，我最初以為，sys.exit(0)---正常退出腳本，sys.exit(1)非正常退出腳本，但是實際情況是/9sys.exit(1)，僅輸出返回碼不同）：

復(fù)制代碼代碼如下:

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
>>>

由此可見，這樣做沒有達到我預(yù)期如下輸出的效果，那么，問題在哪里呢？在于sys.exit()始終會拋出一個SystemExit異常。

復(fù)制代碼代碼如下:

Input your words,please!_d4%

_d4% isn't legal identifier for Python!

復(fù)制代碼代碼如下:

#!/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()

上面的代碼獲取sys.exit()拋出的SystemExit異常。

return：在定義函數(shù)時從函數(shù)中返回一個函數(shù)的返回值，終止函數(shù)的執(zhí)行。

exit:下面的代碼中，如果把sys.exit()替換成exit，則exit僅僅跳出離它最近的for循環(huán)， print "%s is legal identifier for Python!2" % idInput語句會被輸出，這里，exit的作用類似于break. 但實際上break和exit作用并不同

復(fù)制代碼代碼如下:

                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

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