Re-raise different exception with original stack trace

[MRAB]

On 14/10/2011 22:30, Prasad, Ramit wrote:
> Hi all,
> Hopefully you guys can help me with my problem.
>
> Basically I have a UI program that can "save" information. The UI passes the save to the controller and the controller saves a file and does some post processing. If saving the file fails, I want to handle the error differently than if the error was in post processing. In both cases the possible exceptions are varied and unknown so I cannot pick and choose certain exception. The problem with the sample program below is that the stack trace is replaced by the re-raise. What would be the best way(s) to get the original stack trace in save_from_UI if the exception occurs in post_process? Using Python 2.6 and Windows(XP and 7) / Linux.
>
> Def save_from_model():
>      save() # do not catch exception (could be any exception)
>      try:
>          post_process()
>      except Exception as e:
>          #do something
>          raise CustomException() # "wrap" exception so bar knows to handle it differently
>
> def save_from_UI():
>      try:
>          foo()
>      except CustomException() as e:
>          # How do I get the original stacktrace instead of the reraise?
>      except Exception as e:
>          # do something else with this exception
>
You could save a reference to the exception in the custom exception:

class CustomException(Exception):
     def __init__(self, e):
         Exception.__init__(self)
         self.exc = e

def save_from_model():
     save() # do not catch exception (could be any exception)
     try:
         post_process()
     except Exception as e:
         #do something
         raise CustomException(e)

def save_from_UI():
     try:
         foo()
     except CustomException as e:
         # Original exception is e.exc
     except Exception as e:
         # do something else with this exception