Re: try/except KeyError vs "if name in ..."

Newsgroups	comp.lang.python
Date	2012-10-06 21:17 -0700
References	<k4oj0d$qgl$1@thue.elzevir.fr> <507017AB.8090507@davea.name> <mailman.1909.1349552576.27098.python-list@python.org>
Subject	Re: try/except KeyError vs "if name in ..."
From	Ramchandra Apte <maniandram01@gmail.com>
Message-ID	<mailman.1916.1349583456.27098.python-list@python.org> (permalink)

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On Sunday, 7 October 2012 01:12:56 UTC+5:30, Terry Reedy  wrote:
> On 10/6/2012 7:36 AM, Dave Angel wrote:
> 
> 
> 
> > The distinction in performance between the success and failure modes of
> 
> > the try/catch isn't nearly as large as one of the other responses might
> 
> > lead you to believe.  For example, a for loop generally terminates with
> 
> > a raise (of StopIteration exception), and that doesn't convince us to
> 
> > replace it with a while loop.
> 
> 
> 
> For statement generally loop many times, up to millions of times, 
> 
> without an exception being raised, whereas while statements test the 
> 
> condition each time around the loop. So the rule 'if failure is rare 
> 
> (less than 10-20%) use try', applies here. For if/them versus 
> 
> try/except, I don't worry too much about it.
> 
> 
> 
> -- 
> 
> Terry Jan Reedy

I use try and except when I need to raise exceptions e.g.:

try:
    value = vm.variables[name]
except KeyError:
    raise NameError("variable name not defined in VM's variables")

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Thread

try/except KeyError vs "if name in ..." Manuel Pégourié-Gonnard <mpg@elzevir.fr> - 2012-10-06 08:27 +0200
  Re: try/except KeyError vs "if name in ..." Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-06 08:33 +0000
    Re: try/except KeyError vs "if name in ..." Manuel Pégourié-Gonnard <mpg@elzevir.fr> - 2012-10-06 12:08 +0200
  Re: try/except KeyError vs "if name in ..." "Günther Dietrich" <gd.usenet@spamfence.net> - 2012-10-06 10:49 +0200
    Re: try/except KeyError vs "if name in ..." Manuel Pégourié-Gonnard <mpg@elzevir.fr> - 2012-10-06 12:09 +0200
  Re: try/except KeyError vs "if name in ..." Dave Angel <d@davea.name> - 2012-10-06 07:36 -0400
  Re: try/except KeyError vs "if name in ..." Terry Reedy <tjreedy@udel.edu> - 2012-10-06 15:42 -0400
    Re: try/except KeyError vs "if name in ..." Ramchandra Apte <maniandram01@gmail.com> - 2012-10-06 21:17 -0700
    Re: try/except KeyError vs "if name in ..." Ramchandra Apte <maniandram01@gmail.com> - 2012-10-06 21:17 -0700

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