使用Python引发异常时回滚操作的最佳方法

Best way to roll back actions when an exception is raised with Python

我有一个这样的代码：

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# step 1 remove from switch
for server in server_list:
remove_server_from_switch(server)
logger.info("OK : Removed %s", server)

# step 2 remove port
for port in port_list:
remove_ports_from_switch(port)
logger.info("OK : Removed port %s", port)

# step 3 execute the other operations
for descr in pairs:
move_descr(descr)

# step 4 add server back to switch
for server in server_list:
add_server_to_switch(server)
logger.info("OK : server added %s", server)

# step 5 add back port
for port in port_list:
add_ports_to_switch(port)
logger.info("OK : Added port %s", port)

for循环中的函数可以引发异常，或者用户可以使用ctrl+c中断脚本。但是，如果在执行过程中引发异常，我希望通过撤消之前已经完成的更改来进入回滚模式。我的意思是，如果在步骤3中出现异常，我必须回滚步骤1和2(通过执行步骤4和5中的操作)。或者，如果用户试图在步骤1的for循环中间使用ctrl+c停止脚本，我希望回滚该操作并添加已删除的服务器。

除了使用例外情况，如何才能用一种好的Python式的方式来做呢？：)

这就是上下文管理器的作用。详细阅读WITH语句，但总的来说，您需要编写上下文管理器类，其中__enter__和__exit__函数用于删除/重新添加服务器/端口。然后您的代码结构就变成了：

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with RemoveServers(server_list):
with RemovePorts(port_list):
do_stuff
# exiting the with blocks will undo the actions

也许这样的方法会奏效：

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undo_dict = {remove_server_from_switch: add_server_to_switch,
remove_ports_from_switch: add_ports_to_switch,
add_server_to_switch: remove_server_from_switch,
add_ports_to_switch: remove_ports_from_switch}

def undo_action(action):
args = action[1:]
func = action[0]
undo_dict[func](*args)

try:

#keep track of all successfully executed actions
action_list = []

# step 1 remove from switch
for server in server_list:
remove_server_from_switch(server)
logger.info("OK : Removed %s", server)
action_list.append((remove_server_from_switch, server))

# step 2 remove port
for port in port_list:
remove_ports_from_switch(port)
logger.info("OK : Removed port %s", port)
action_list.append((remove_ports_from_switch, port))

# step 3 execute the other operations
for descr in pairs:
move_descr(descr)

# step 4 add server back to switch
for server in server_list:
add_server_to_switch(server)
logger.info("OK : server added %s", server)
action_list.append((add_server_to_switch, server))

# step 5 add back port
for port in port_list:
add_ports_to_switch(port)
logger.info("OK : Added port %s", port)
action_list.append((add_ports_to_switch, port))

except Exception:
for action in reverse(action_list):
undo_action(action)
logger.info("ERROR Recovery : undoing {func}({args})",func = action[0], args = action[1:])

finally:
del action_list

编辑：正如Tzaman下面所说，在这种情况下，最好的做法是将整个事情包装成一个上下文管理器，并使用with语句。然后，不管是否遇到错误，您的所有操作在with块结束时都将被撤消。

下面是它可能的样子：

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class ActionManager():
def __init__(self, undo_dict):
self.action_list = []
self.undo_dict = undo_dict
def action_pop(self):
yield self.action_list.pop()
def action_add(self, *args):
self.action_list.append(args)
def undo_action(self, action):
args = action[1:]
func = action[0]
self.undo_dict[func](*args)
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
for action in self.action_stack:
undo_action(action)
logger.info("Action Manager Cleanup : undoing {func}({args})",func = action[0], args = action[1:])

现在你可以这样做了：

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#same undo_dict as before
with ActionManager(undo_dict) as am:

# step 1 remove from switch
for server in server_list:
remove_server_from_switch(server)
logger.info("OK : Removed %s", server)
am.action_add(remove_server_from_switch, server)

# step 2 remove port
for port in port_list:
remove_ports_from_switch(port)
logger.info("OK : Removed port %s", port)
am.action_add(remove_ports_from_switch, port)

# step 3 execute the other operations
for descr in pairs:
move_descr(descr)

# steps 4 and 5 occur automatically

另一种方法是在__enter__方法中添加服务器/端口，这可能会更好。您可以将上面的ActionManager子类化，并在其中添加端口添加和删除逻辑。

__enter__方法甚至不必返回ActionManager类的一个实例——如果这样做是有意义的，甚至可以编写它，以便with SwitchManager(servers,ports)返回pairs对象，最后可以这样做：

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with SwitchManager(servers, ports) as pairs:
for descr in pairs:
move_descr(descr)