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Groups > comp.lang.python > #48858

Re: Default Value

From "Lefavor, Matthew (GSFC-582.0)[MICROTEL LLC]" <matthew.lefavor@nasa.gov>
Date 2013-06-20 17:28 -0500
Subject Re: Default Value
References (2 earlier) <447dd1c6-1bb2-4276-a109-78d7a067b442@d8g2000pbe.googlegroups.com> <2e92b4c7-31be-40d2-a906-ab19f3630dfa@googlegroups.com> <09cfa648-ab87-4261-b8f0-bf493de00553@4g2000pbf.googlegroups.com> <CAPTjJmr4Ny-0yKptf+_6QKrKoa67j1UsfYsJ_SqaPbYZqA6mQQ@mail.gmail.com> <CALwzid=ZK+a3++izA7=DmWcciYeKn7a55DQBUVBVmsW_LRDqWA@mail.gmail.com>
Newsgroups comp.lang.python
Message-ID <mailman.3662.1371812413.3114.python-list@python.org> (permalink)

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Or, in many MANY cases, the choice was the right one, but isn't
obvious to everyone.

I think it's worth pointing out that changing function defaults to
late-binding would merely change the nature of the gotcha, not eliminate it.

words = ("one", "two", "red", "blue", "fish")

def join_strings(strings=words):
   return ' '.join('%s %s' % (s, strings[-1]) for s in strings[:-1])

# Later:
words = open("gettysburg_address.txt", "r").read().split()
# Oops, the default argument of join_strings just changed.

+1. This is what convinces me that keeping references to keyword arguments is actually the right thing to do.

Perhaps I'm biased because I'm used to the mutable-argument behavior by now, but the gotcha described above seems to be much harder to track down and understand than any bugs caused by mutable arguments. With mutable arguments, at least you know the cause is in the function definition. If you initialized variables each time, you'd have to track the problem down across the entire namespace.

I, for one, would much rather follow the rule "don't use mutable arguments in function definitions" than "don't ever re-use the name of your variables."

ML

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Default Value Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-06-19 12:17 -0700
  Re: Default Value Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-06-19 22:35 +0300
  Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-19 12:38 -0700
    Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-20 05:57 -0700
      Re: Default Value Roy Smith <roy@panix.com> - 2013-06-20 09:19 -0400
        Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-20 06:31 -0700
        Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 00:17 +0000
          Re: Default Value Roy Smith <roy@panix.com> - 2013-06-20 20:25 -0400
      Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-20 07:49 -0700
        Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-21 01:38 +1000
          Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-20 11:05 -0700
            Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 00:57 +0000
              Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-20 20:16 -0700
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-21 17:10 +1000
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 07:32 -0700
                Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 16:22 +0000
            Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 15:57 +0000
              Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 10:01 -0700
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-21 18:47 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 11:26 -0700
                Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 18:37 +0000
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 12:18 -0700
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 12:35 -0700
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-22 05:07 +1000
                Re: Default Value Neil Cerutti <neilc@norwich.edu> - 2013-06-21 19:20 +0000
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 13:27 -0700
                Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-22 01:31 +0000
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-22 11:40 +1000
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 18:42 -0700
                Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-22 03:04 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 19:32 -0700
                Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-22 20:36 +0100
                Re: Default Value Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-06-22 18:51 -0400
                Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-21 19:43 -0700
                Re: Default Value Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-06-22 09:26 +0100
                Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-21 20:25 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 13:44 -0700
                Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-21 23:49 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 16:51 -0700
                Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-22 02:54 +0100
                Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-22 01:15 +0000
                Re: Default Value Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-21 19:27 -0600
                Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-21 19:32 -0700
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 19:55 -0700
                Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-21 23:23 -0700
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-22 08:07 -0700
                Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-22 08:31 -0700
                Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-22 09:11 -0700
                Re: Default Value Grant Edwards <invalid@invalid.invalid> - 2013-06-24 14:22 +0000
                Re: [SPAM] Re: Default Value MRAB <python@mrabarnett.plus.com> - 2013-06-24 16:22 +0100
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-22 11:41 +1000
                Re: Default Value Michael Torrie <torriem@gmail.com> - 2013-06-21 20:46 -0600
                Re: Default Value Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-06-22 16:40 +0200
                Re: Default Value Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-06-22 12:49 -0400
                Re: Default Value Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-22 12:57 -0600
                Re: Default Value Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-06-22 18:48 -0400
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-22 00:40 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 18:15 -0700
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-22 11:37 +1000
                Re: Default Value Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-21 19:42 -0600
                Re: Default Value Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-21 19:38 -0600
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 19:40 -0700
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-22 03:01 +0100
                Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-22 12:18 +1000
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-22 03:25 +0100
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-22 18:19 +0100
                Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-22 11:49 -0700
                Re: Default Value Rotwang <sg552@hotmail.co.uk> - 2013-06-23 01:49 +0100
        Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-20 10:12 -0700
          Re: Default Value Chris Angelico <rosuav@gmail.com> - 2013-06-21 03:19 +1000
            Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-20 10:30 -0700
          Re: Default Value Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-20 11:57 -0600
            Re: Default Value rusi <rustompmody@gmail.com> - 2013-06-20 11:15 -0700
            Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 01:08 +0000
              Re: Default Value Tim Chase <python.list@tim.thechases.com> - 2013-06-20 20:26 -0500
                Re: Default Value Roy Smith <roy@panix.com> - 2013-06-20 21:40 -0400
                Re: Default Value Tim Chase <python.list@tim.thechases.com> - 2013-06-20 21:02 -0500
          Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-20 11:07 -0700
            Re: Default Value alex23 <wuwei23@gmail.com> - 2013-06-21 10:55 +1000
          Re: Default Value 88888 Dihedral <dihedral88888@gmail.com> - 2013-06-20 19:18 -0700
          Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 02:26 +0000
          Re: Default Value "Lefavor, Matthew (GSFC-582.0)[MICROTEL LLC]" <matthew.lefavor@nasa.gov> - 2013-06-20 17:28 -0500
            Re: Default Value Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-21 08:17 -0700
        Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-21 01:28 +0000
  Re: Default Value Gary Herron <gherron@digipen.edu> - 2013-06-19 12:57 -0700
  Re: Default Value Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-06-20 01:17 +0000

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