Python：在try/except/else塊中引發異常，處理順序問題

Question

我是Python的新手，並開始編寫我的第一個模塊，它將對外部存儲（通常是一個或多個USB磁盤）執行備份。

的期望的行爲是：

1. 檢查如果目的地（備份磁盤）已經安裝。 destination_was_mounted變成True或False
2. 如果destination_was_mounted = False，則裝入目標。
3. 如果mountDestination失敗，引發異常並重新啓動循環。
4. 如果mountDestination正常，請檢查目標是否存在checkfile（和backup_dir）。
5. 如果check_destination失敗引發異常並重新啓動循環。
6. 繼續處理（尚未編碼此）。
7. 在任何情況下，如果destination_was_mounted = False，則卸載目標。

問題是，如果check_destination部分引發異常，它將無法卸載目標，即使我已經在最後一節。就好像destination_was_mounted變成True，即使它應該是False。或者就好像check_destination在mount_destination之前運行，即使它在它之後。

我引用（除了解一下Python文檔和我的Python入門書）：

Python: How to tell the for loop to continue from a function?

How to retry after exception in python?

How to get back to the for loop after exception handling

#!/usr/bin/env python3.1 

import sys 
import os 
import os.path 
import subprocess 
import configparser 
CONFIGFILE = 'backup.ini' 
# do i need this? 
config = {} 

config = configparser.ConfigParser() 
config.read(CONFIGFILE) 
backup_sources = sorted(config.sections()[1:]) 

class NoCheckFile(Exception): 
    pass 

def mountDestination(destination): 
    return subprocess.check_call(['mount', destination]) 

def unMountDestination(destination): 
    return subprocess.check_call(['umount', destination]) 

def checkDestination(destination, backup_dir, checkfile): 
    return os.path.exists(destination + '/' + backup_dir + '/' + checkfile) 

''' exception handlers ''' 
# perhaps add another arg like 0 or 1 for success/failure 
def handleCalledProcessError(ex): 
    print('Exception: ' + str(ex)) 

def handleNoCheckFile(ex): 
    print('Exception: ' + str(ex)) 

# rename me once I work out logging 
def logExecute(result): 
    print('Info: ' + str(result)) 
# can I pass logging output here 

def main(): 
    for section in backup_sources: 
     item = dict(config.items(section)) 
     destination = item['destination'] 
     destination_was_mounted = os.path.ismount(destination) 
     backup_dir = item['backup_dir'] 
     checkfile = item['checkfile'] 
     try: 
      ''' check destination_was_mounted and mount destination if required ''' 
      mount_destination = None 
      unmount_destination = None 
      if not destination_was_mounted: 
       mount_destination = mountDestination(destination) 

      ''' check that checkfile exists in backup_dir ''' 
      check_destination = checkDestination(destination, backup_dir, checkfile) 
      if not check_destination: 
       raise NoCheckFile('no checkfile found') 

      ''' lvm snapshot, mount and source path update ''' 

      ''' backup engine code here ''' 

      ''' check destination_was_mounted and um-mount destination if required ''' 
      if not destination_was_mounted: 
       unmount_destination = unMountDestination(destination) 
     except subprocess.CalledProcessError as ex: 
      print(destination, 'mounted before loop start: ', destination_was_mounted) 
      handleCalledProcessError(ex) 
     except NoCheckFile as ex: 
      handleNoCheckFile(ex) 
     else: 
      print(destination, 'mounted before loop start: ', destination_was_mounted) 
      logExecute(mount_destination) 
      logExecute(check_destination) 
     finally: 
      print('should always see me') 
      logExecute(unmount_destination) 
      # return to say True or False 

    # this should be where email reports etc. go 

if __name__ == '__main__': 
    main() 

的backup.ini的相關部分文件是：

[general] 

[1] 
DESTINATION = /mnt/backup2 
BACKUP_DIR = BACKUP2 
CHECKFILE = .checkfile 

[2] 
DESTINATION = /mnt/backup1 
BACKUP_DIR = BACKUP1 
CHECKFILE = .checkfile 

輸出看起來像這樣 - 我在[1]和[2]中指定的掛載點處附加了2個備份磁盤，並且故意沒有爲[1]創建檢查文件來測試。

> umount /mnt/backup1 
umount: /mnt/backup1: not mounted 
> umount /mnt/backup2 
umount: /mnt/backup2: not mounted 
> mugsyback.py 
Exception: no checkfile found 
should always see me 
Info: None 
/mnt/backup1 mounted before loop start: False 
Info: 0 
Info: True 
should always see me 
Info: 0 
> mount 
... 
/dev/sdc1 on /mnt/backup2 type ext3 (rw) 

Answer 1

您在try塊中有卸載代碼。最後，你只是在登錄。