[Tutor] Why subclassing exceptions?

[Mac Ryan]

On Wed, 28 Sep 2011 09:58:49 +1000
Steven D'Aprano <steve at pearwood.info> wrote:

> Mac Ryan wrote:
> 
> >     raise BaseException('Something is wrong here!')  
> 
> Never raise BaseException directly! BaseException is the very top of
> the exception hierarchy, you should raise the *most* specific
> exception you can, not the least specific. BaseException isn't even
> just for errors, it's also for flow control exceptions like
> StopIteration .

Fair enough! :) I just put that line to signal that there were cases
that weren't catch by the series of ``if``, but nevertheless you are
right.

**I would like to know more on the rationale behind this
guidelines, though.**

I explain: I already knew before that I should
subclass exceptions, nevertheless I very seldom do, as I feel like they
are just taking space on my screen doing nothing useful. All I need to
do is often just print out a useful message, so I find that

>>> raise StandardError('This value should be True.')

does the job in a much more elegant way than:

>>> class ShouldBeTrueError(StandardError):
...     
...     '''
...     Custom Error triggered in case a value should be True. 
...     '''
...     
...     pass
... 
>>> raise ShouldBeTrueError('Doh! An error!')

Ok, I already hear some of you screaming out loud "Anathema!!" ;) ...yet
could somebody please clarify why should I bother subclassing? [I'm
positive there is a good reason, but in all honesty I can't imagine
which one...].

Thanks,
/mac