change an exception's message and re-raise it

[Steven D'Aprano]

On Thu, 31 Dec 2009 12:18:42 -0800, Phlip wrote:

> Pythonistas:
> 
> I need to do this:
> 
>   try:
>      deep_arcane_layer()
>   except e:
>     e.message = 'the deep arcane layer says: ' + e.message
>     raise e

Use e.args, not e.message. The message attribute is deprecated from 
Python 2.6 and will print a warning if you try to use it.


> The point is I need to augment that layer's exceptions with extra
> information that I know about that layer.
> 
> I naturally cannot use the argless version of 'raise', because it only
> re-raises whatever exception object is currently in play - and it
> appears to be read-only or locked or something.

Changing args works:


>>> try:
...     1/0
... except ZeroDivisionError, e:
...     e.args = e.args + ('fe', 'fi', 'fo', 'fum')
...     raise
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: ('integer division or modulo by zero', 'fe', 'fi', 
'fo', 'fum')




> I also should not do this...
> 
>    raise Exception('blah ' + e.message)
> 
> ...because that stripped off the exception type itself, and higher
> layers need to know this.

For the record you can get the exception type from type(e):

raise type(e)("whatever you want")

but that creates a new exception, not re-raising the old one.




-- 
Steven