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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

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Thread

Re: Confused compare function :) Bruno Dupuis <python.ml.bruno.dupuis@lisael.org> - 2012-12-06 01:19 +0100
  Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 00:42 +0000
    Re: Confused compare function :) Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-12-06 13:41 -0500
    Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 19:55 +0100
  Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 03:22 +0000
    Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 04:32 +0000
      Re: Confused compare function :) Bruno Dupuis <python.ml.bruno.dupuis@lisael.org> - 2012-12-06 09:49 +0100
        Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 11:47 +0000
          Re: Confused compare function :) peter <pjmakey2@gmail.com> - 2012-12-06 08:55 -0300
            Re: Confused compare function :) Hans Mulder <hansmu@xs4all.nl> - 2012-12-06 14:32 +0100
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 00:47 +1100
          Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-06 23:14 +1100
            Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-07 22:16 +0000
              Re: Confused compare function :) Terry Reedy <tjreedy@udel.edu> - 2012-12-08 02:01 -0500
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-08 18:17 +1100
              Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-08 17:50 +0000
            Re: Confused compare function :) Ramchandra Apte <maniandram01@gmail.com> - 2012-12-08 19:07 -0800
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-09 14:22 +1100
                Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-09 07:39 +0000
            Re: Confused compare function :) Ramchandra Apte <maniandram01@gmail.com> - 2012-12-08 19:07 -0800
          Re: Confused compare function :) Neil Cerutti <neilc@norwich.edu> - 2012-12-06 13:51 +0000
            Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-07 02:55 +0000
              Re: Confused compare function :) Neil Cerutti <neilc@norwich.edu> - 2012-12-07 16:40 +0000
        Re: Confused compare function :) Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-12-06 14:33 +0100
          Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 00:58 +1100
            Re: Confused compare function :) Hans Mulder <hansmu@xs4all.nl> - 2012-12-06 15:21 +0100
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 01:28 +1100
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 15:22 +0100
          Re: Confused compare function :) Dave Angel <d@davea.name> - 2012-12-06 09:40 -0500
          Re: Confused compare function :) peter <pjmakey2@gmail.com> - 2012-12-06 11:46 -0300
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 17:16 +0100
          Re: Confused compare function :) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-12-06 16:52 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 18:08 +0100
          Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-06 17:10 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 18:31 +0100
          Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-06 17:52 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 19:25 +0100
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-07 14:36 +0100
        Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 19:24 +0000
      Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-06 20:34 +1100
      Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 19:25 +0000

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