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Re: Confused compare function :)

References (1 earlier) <mailman.538.1354753193.29569.python-list@python.org> <k9p32p$nek$1@dont-email.me> <50c01fe2$0$21853$c3e8da3$76491128@news.astraweb.com> <mailman.549.1354783761.29569.python-list@python.org> <k9q6s4$os9$1@r03.glglgl.gl>
Date 2012-12-07 00:58 +1100
Subject Re: Confused compare function :)
From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Message-ID <mailman.557.1354802285.29569.python-list@python.org> (permalink)

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On Fri, Dec 7, 2012 at 12:33 AM, Thomas Rachel
<nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de>
wrote:
> Am 06.12.2012 09:49 schrieb Bruno Dupuis:
>
>> 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
>
> I would do
>
>     try:
>         value = mydict[k]
>     except KeyError:
>         pass
>     else:
>         do_stuff(k)
>
> Why? Because do_stuff() might raise a KeyError, which should not go
> undetected.

(Assuming first off that you meant "do_stuff(value)", not
"do_stuff(k)", in that last line)

That has quite different functionality, though. The original wouldn't
have called do_stuff at all if k is not in dict, behaviour which is
matched by both his EAFP and his LBLY. But your version, in the event
of a KeyError, will call do_stuff with the previous value of value, or
raise NameError if there is no such previous value. I don't think
that's intentional.

The only way around it that I can see is an extra condition or jump -
something like:

def call_if_present(mydict,k,do_stuff):
    """Equivalent to
    do_stuff(mydict[k])
    if the key is present; otherwise, does not call do_stuff, and
returns None."""
    try:
        value = mydict[k]
    except KeyError:
        return
    return do_stuff(value)

It'll propagate any other exceptions from the subscripting (eg
TypeError if you give it a list instead of a dict), and any exceptions
from do_stuff itself. But it's getting a bit unwieldy.

ChrisA

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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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