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Date: Thu, 6 Dec 2012 09:49:26 +0100
From: Bruno Dupuis <python.ml.bruno.dupuis@lisael.org>
To: python-list@python.org
Subject: Re: Confused compare function :)
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On Thu, Dec 06, 2012 at 04:32:34AM +0000, Steven D'Aprano wrote:
> On Thu, 06 Dec 2012 03:22:53 +0000, Rotwang wrote:
> 
> > On 06/12/2012 00:19, Bruno Dupuis wrote:
> >> [...]
> >>
> >> Another advice: never ever
> >>
> >> except XXXError:
> >>      pass
> >>
> >> at least log, or count, or warn, or anything, but don't pass.
> > 
> > Really? I've used that kind of thing several times in my code. For
> > example, there's a point where I have a list of strings and I want to
> > create a list of those ints that are represented in string form in my
> > list, so I do this:
> > 
> > listofints = []
> > for k in listofstrings:
> > 	try:
> > 		listofints.append(int(k))
> > 	except ValueError:
> > 		pass
> > 
> > Another example: I have a dialog box with an entry field where the user
> > can specify a colour by entering a string, and a preview box showing the
> > colour. I want the preview to automatically update when the user has
> > finished entering a valid colour string, so whenever the entry field is
> > modified I call this:
> > 
> > def preview(*args):
> > 	try:
> > 		previewbox.config(bg = str(entryfield.get()))
> > 	except tk.TclError:
> > 		pass
> > 
> > Is there a problem with either of the above? If so, what should I do
> > instead?
> 
> They're fine.
> 
> Never, ever say that people should never, ever do something.
> 
> 
> *cough*
> 

Well, dependening on the context (who provides listofstrings?) I would
log or count errors on the first one... or not.

On the second one, I would split the expression, because (not sure of
that point, i didn't import tk for years) previewbox.config and
entryfield.get may raise a tk.TclError for different reasons.

The point is Exceptions are made for error handling, not for normal
workflow. I hate when i read that for example:

    try:
        do_stuff(mydict[k])
    except KeyError:
        pass

(loads of them in many libraries and frameworks)
instead of:

    if k in mydict:
        do_stuff(mydict[k])

Note that the performances are better with the latter.

There are some exceptions to this, though, like StopIteration

For me, it's a rule of thumb, except: pass is possible in situations
where I control every input data, and I deeply, exactly know all code
interractions. If figuring all this out is longer (it's almost always
the case) than typing:

log.warning('oops:\n %s' % traceback.format_exc())

I log. 

It depends also on the context, I'd be more 'permissive' a short
script than into a large program, framework, or lib, for the
very reason it's easy to know all code interactions.

In my coder life, i spent more time debugging silently swallowed exceptions
than logging abnormal behaviours.

-- 
Bruno Dupuis