Modify an exception before re-raising it

[Steve Holden]

Steven D'Aprano wrote:
> I wish to catch an exception, modify the error message, and re-raise it.
> There are two ways I know of to do this, with subtly different effects:
> 
>>>> def raise_example1():
> ...     try:
> ...             None()
> ...     except TypeError, e:
> ...             e.args = ('modified error message',) + e.args[1:]
> ...             raise e
> ... 
>>>> def raise_example2():
> ...     try:
> ...             None()
> ...     except TypeError, e:
> ...             e.args = ('modified error message',) + e.args[1:]
> ...             raise
> ... 
>>>> raise_example1()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "<stdin>", line 6, in raise_example1
> TypeError: modified error message
>>>> raise_example2()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "<stdin>", line 3, in raise_example2
> TypeError: modified error message
> 
> Note how the line numbers in the traceback are different.
> 
> The behaviour I want is from raise_example2, but I'm not sure if this is
> documented behaviour, or if it is something I can rely on. Is it acceptable
> to modify an exception before re-raising it?
> 
> 
Yes, "raise" on its own is specifically documented (he says, without
consulting the documentation - but then if you didn't, why should I?) to
re-raise a current exception. I see no reason why you shouldn't modify
your exception before the re-raise. You certainly don't want to raise
the exception explicitly, since as you have just discovered that will
alter its traceback.

regards
 Steve
-- 
Steve Holden           +1 571 484 6266   +1 800 494 3119
Holden Web LLC                 http://www.holdenweb.com/
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