Subclassing exceptions

[Chris Liechti]

sag at hydrosphere.com (Sue Giller) wrote in 
news:mailman.1010102348.1522.python-list at python.org:
> I am trying to write an exception class that will pass on info from a 
> previous exception (if any) along with the currently raised exception.

why do you wanna do this?
i often use:
...
except:
    print >>sys.stderr, "Exception in _ComPortThread:"
    traceback.print_exc(file=sys.stderr)

to print a tarceback but continue the program.

> Here is the exception class (test case):  It wants to append any input 
> args to any existing text from an exception.
> 
> import exceptions
> class Ex(exceptions.Exception):    
>     def __init__(self, args=None):
>         import sys                
>         info = sys.exc_info()
>         text = ""
>         if not info is None:               # always get this!
>             text = "%s" % str(info[1])
>         if args != None:
>             text = text + "\n\t" + args
>         self.args = text

i don't know how secure is it to get exception info during an exception is 
raised... i would leave my fingers of it...

i would process the last exception end then raise the own exception along 
with the extracted info.
 
> Is there some way to write a subclassed exception that allows me to 
> pass on the text from the last raised exception (if any) plus my 
> subclassed text?

does this work for you:
class MyException(Exception): pass
...
try:
...
except Exception, msg:
   raise MyException, msg
...

> This does not happen from the interactive prompt, but will happen 
> from a script.  You can have a simple script in PythonWin, with just 
> the lines:
> 
> import sys
> print sys.exc_info()
> 
> and get the same results
> 
> ('win32ui', 'There is no active view.', <traceback object at 
> 0133FF70>)

python win uses python too.. so maybe you see an exception that was 
generated in the GUI.

-- 
Chris <cliechti at gmx.net>