[Tutor] How to raise error without the stack trace

[Steven D'Aprano]

Hugo Arts wrote:
> On Sat, Nov 26, 2011 at 11:16 AM, Karim <kliateni at gmail.com> wrote:
>> Hello,
>>
>> I want to fire my own exception without having the (useful but ugly in my
>> case) stack trace display.
>>
>> How to modify a Exception type class for that purpose which looks like:
>>
>> classs MyError(Exception):
>>       pass
>>
>> Cheers
>> Karim
>>
> 
> The stack trace is what happens when an exception is uncaught. You
> don't need to modify it. What you want to do is catch the exception,
> print it, then, depending on whether you can recover from the error,
> possibly do a sys.exit(-1).

If you can recover from the error, by all means catch the exception. That's 
what exceptions are for!

But otherwise, please think very, very, VERY carefully before printing an 
error message and exiting. This is normally the wrong thing to do.

Stack traces are useful. They show you exactly what has gone wrong, and where 
(although sadly not why!). There is very little worse than a "helpful" 
programmer who turns a sensible, useful stack trace into a useless, stupid 
message:

"An error has occurred"

or worse.

Perhaps the most sensible example I can think of is this:


def main():
     # your code goes here...


if __name__ == '__main__':
     try:
         main()
     except KeyboardInterupt:
         # Not an error, so no need for a stack trace.
         print "operation cancelled by user"
         sys.exit(1)


and that's it. Any other exception is a bug in your code, and so the stack 
trace should be printed so the user can report it.


-- 
Steven