Re: what does 'a=b=c=[]' do

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From	Ian Kelly <ian.g.kelly@gmail.com>
Date	2011-12-22 19:59 -0700
Subject	Re: what does 'a=b=c=[]' do
Newsgroups	comp.lang.python
Message-ID	<mailman.4013.1324609184.27778.python-list@python.org> (permalink)

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On Thu, Dec 22, 2011 at 7:10 PM, alex23 <wuwei23@gmail.com> wrote:
> On Dec 22, 6:51 pm, Rolf Camps <r...@roce.be> wrote:
>> I'm afraid it's dangerous to encourage the use of '[]' as assignment to
>> a parameter in a function definition. If you use the function several
>> times 'default' always points to the same list.
>
> I appreciate the concern, but adding a default argument guard would
> not only obscure the code. It's irrelevant, as you recognise, because
> no matter what, it's going to make copies of the default argument.

It's only irrelevant in the immediate context of the code you posted.
But when Joe Novice sees your code and likes it and duplicates it a
million times without receiving any warning about it, he's eventually
going to write a function that modifies its default list argument, and
he'll be in for a nasty surprise when he does.

Thread

what does 'a=b=c=[]' do Eric <einazaki668@yahoo.com> - 2011-12-21 14:25 -0800
  Re: what does 'a=b=c=[]' do Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-12-21 18:20 -0500
    Re: what does 'a=b=c=[]' do Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-21 23:48 +0000
      Re: what does 'a=b=c=[]' do Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-12-24 19:41 +0100
        Re: what does 'a=b=c=[]' do Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-25 13:16 +0000
    Re: what does 'a=b=c=[]' do Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-12-24 19:45 +0100
  Re: what does 'a=b=c=[]' do Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-21 23:44 +0000
    Re: what does 'a=b=c=[]' do Eric <einazaki668@yahoo.com> - 2011-12-22 20:27 -0800
  Re: what does 'a=b=c=[]' do alex23 <wuwei23@gmail.com> - 2011-12-21 16:50 -0800
    Re: what does 'a=b=c=[]' do Rolf Camps <rolf@roce.be> - 2011-12-22 09:51 +0100
      Re: what does 'a=b=c=[]' do alex23 <wuwei23@gmail.com> - 2011-12-22 18:10 -0800
        Re: what does 'a=b=c=[]' do Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-22 19:59 -0700
          Re: what does 'a=b=c=[]' do alex23 <wuwei23@gmail.com> - 2011-12-22 19:40 -0800
            Re: what does 'a=b=c=[]' do Chris Angelico <rosuav@gmail.com> - 2011-12-23 15:25 +1100
            Re: what does 'a=b=c=[]' do Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-22 22:22 -0700
              Re: what does 'a=b=c=[]' do alex23 <wuwei23@gmail.com> - 2011-12-22 22:00 -0800
        Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 00:38 -0800
          Re: what does 'a=b=c=[]' do Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-23 09:39 +0000
            Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 02:22 -0800
              Re: what does 'a=b=c=[]' do Robert Kern <robert.kern@gmail.com> - 2011-12-23 13:10 +0000
                Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 05:23 -0800
                Re: what does 'a=b=c=[]' do Robert Kern <robert.kern@gmail.com> - 2011-12-23 13:53 +0000
                Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 06:57 -0800
                Re: what does 'a=b=c=[]' do Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-23 15:33 +0000
                Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 07:59 -0800
          Re: what does 'a=b=c=[]' do Ethan Furman <ethan@stoneleaf.us> - 2011-12-23 00:49 -0800
          Re: what does 'a=b=c=[]' do Chris Angelico <rosuav@gmail.com> - 2011-12-23 20:59 +1100
            Re: what does 'a=b=c=[]' do rusi <rustompmody@gmail.com> - 2011-12-23 02:31 -0800
              Timeout when calling COM objects on Windows Wong Wah Meng-R32813 <r32813@freescale.com> - 2011-12-23 11:20 +0000
              Re: what does 'a=b=c=[]' do Michael Torrie <torriem@gmail.com> - 2011-12-23 10:23 -0700
            Re: what does 'a=b=c=[]' do Neil Cerutti <neilc@norwich.edu> - 2011-12-23 13:10 +0000
              Re: what does 'a=b=c=[]' do Neil Cerutti <neilc@norwich.edu> - 2011-12-23 13:13 +0000
                Early and late binding [was Re: what does 'a=b=c=[]' do] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-23 15:49 +0000
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Chris Angelico <rosuav@gmail.com> - 2011-12-24 02:55 +1100
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-23 22:32 +0000
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Chris Angelico <rosuav@gmail.com> - 2011-12-24 09:50 +1100
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-24 08:11 +0000
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Roy Smith <roy@panix.com> - 2011-12-23 11:15 -0500
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] alex23 <wuwei23@gmail.com> - 2011-12-24 05:43 -0800
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Mel Wilson <mwilson@the-wire.com> - 2011-12-23 11:27 -0500
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] alex23 <wuwei23@gmail.com> - 2011-12-24 05:52 -0800
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Neil Cerutti <neilc@norwich.edu> - 2011-12-23 17:03 +0000
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-24 08:25 +0000
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] alex23 <wuwei23@gmail.com> - 2011-12-24 06:08 -0800
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-24 18:25 -0500
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] alex23 <wuwei23@gmail.com> - 2011-12-24 16:10 -0800
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-24 19:32 -0500
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] rusi <rustompmody@gmail.com> - 2011-12-24 19:22 -0800
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Lie Ryan <lie.1296@gmail.com> - 2011-12-25 15:12 +1100
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-23 19:24 -0500
                Re: Early and late binding [was Re: what does 'a=b=c=[]' do] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-24 08:26 +0000
        Re: what does 'a=b=c=[]' do Ethan Furman <ethan@stoneleaf.us> - 2011-12-23 00:38 -0800
    Re: what does 'a=b=c=[]' do Ethan Furman <ethan@stoneleaf.us> - 2011-12-22 05:20 -0800
    Re: what does 'a=b=c=[]' do Eric <einazaki668@yahoo.com> - 2011-12-22 19:46 -0800
  Re: what does 'a=b=c=[]' do Lie Ryan <lie.1296@gmail.com> - 2011-12-24 21:30 +1100
  Re: what does 'a=b=c=[]' do Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-12-24 19:49 +0100

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