Python：在try/except/else块中引发异常，处理顺序问题

Question

我刚开始学习Python，正在写我的第一个模块，这个模块的功能是把数据备份到外部存储设备（通常是一个或多个USB硬盘）。

我希望这个模块能按以下步骤工作：

检查备份目标（备份硬盘）是否已经挂载。根据这个检查，destination_was_mounted会变成True或False。
如果destination_was_mounted是False，那就挂载这个目标。
如果挂载目标失败，就抛出异常并重新开始循环。
如果挂载成功，就检查目标上是否存在checkfile（和backup_dir）。
如果检查目标失败，就抛出异常并重新开始循环。
继续处理（这部分我还没写）。
在任何情况下，如果destination_was_mounted是False，就卸载这个目标。

问题是，如果check_destination部分抛出异常，即使我在finally部分有卸载的代码，它也没有卸载目标。就好像destination_was_mounted变成了True，尽管它应该是False。或者就像check_destination在mount_destination之前运行，尽管它实际上是在后面。

我的参考资料（除了查看Python文档和我的《学习Python》书籍）：

Python：如何让for循环从一个函数继续？

如何在Python中异常后重试？

如何在处理异常后回到for循环？

#!/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()

backup.ini文件中相关的部分是：

[general]

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

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

输出看起来是这样的——我有两个备份硬盘，挂载在[1]和[2]指定的点上，并故意没有为[1]创建checkfile来进行测试。

> 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)

异常处理循环控制文件检查代码逻辑挂载重试机制数据备份目标卸载

Python：在try/except/else块中引发异常，处理顺序问题

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