Re: Puzzled by list-appending behavior

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Date	Thu, 26 May 2011 16:58:25 +0100
From	MRAB <python@mrabarnett.plus.com>
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Subject	Re: Puzzled by list-appending behavior
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On 26/05/2011 06:17, Chris Rebert wrote:
> On Wed, May 25, 2011 at 9:46 PM, Uncle Ben<bgreen@nycap.rr.com>  wrote:
>> In playing with lists of lists, I found the following:
>>
>> (In 3.1, but the same happens also in 2.7)
>>
>> list = [1,2,3]
>> list.append ( [4,5,6] )
>
> Note the lack of output after this line. This indicates that
> list.append([4,5,6]) returned None. Contrast this with, say,
> list.pop().
>
>> x = list
>> x   ->
>>     [1,2,3,[4,5,6]]
>> as expected.
>>
>> But the shortcut fails:
>>
>> list=[1,2,3]
>> x = list.append( [4,5,6] )
>> x   ->
>>    nothing
>>
>> Can someone explain this to me?
>
> The append() method does *not* return the now-appended-to list. It is
> a mutator method that modifies the list object in-place; per
> convention, it therefore returns None to reinforce its side-effecting
> nature to the user (the interactive interpreter by default does not
> display None expression results); analogous methods in other languages
> return void.
>
> list.remove(), list.sort(), and list.extend() similarly return None
> rather than the now-modified list.
>
I'd just like to point out that it's a convention, not a rigid rule.
Sometimes it's not followed, for example, dict.setdefault.

Thread

Puzzled by list-appending behavior Uncle Ben <bgreen@nycap.rr.com> - 2011-05-25 21:46 -0700
  Re: Puzzled by list-appending behavior Ben Finney <ben+python@benfinney.id.au> - 2011-05-26 15:11 +1000
  Re: Puzzled by list-appending behavior Chris Rebert <clp2@rebertia.com> - 2011-05-25 22:17 -0700
  Re: Puzzled by list-appending behavior Chris Angelico <rosuav@gmail.com> - 2011-05-26 17:20 +1000
  Re: Puzzled by list-appending behavior Chris Angelico <rosuav@gmail.com> - 2011-05-26 17:23 +1000
  Re: Puzzled by list-appending behavior Uncle Ben <bgreen@nycap.rr.com> - 2011-05-26 00:33 -0700
  Re: Puzzled by list-appending behavior Chris Rebert <clp2@rebertia.com> - 2011-05-26 01:09 -0700
  Re: Puzzled by list-appending behavior MRAB <python@mrabarnett.plus.com> - 2011-05-26 16:58 +0100
    Re: Puzzled by list-appending behavior Tim Roberts <timr@probo.com> - 2011-05-26 23:34 -0700
      Re: Puzzled by list-appending behavior MRAB <python@mrabarnett.plus.com> - 2011-05-27 17:02 +0100
  Re: Puzzled by list-appending behavior Chris Angelico <rosuav@gmail.com> - 2011-05-27 02:04 +1000
  Re: Puzzled by list-appending behavior John Ladasky <ladasky@my-deja.com> - 2011-05-26 11:27 -0700
    Re: Puzzled by list-appending behavior Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-27 01:59 +0000
      Re: Puzzled by list-appending behavior Chris Angelico <rosuav@gmail.com> - 2011-05-27 13:24 +1000
        Re: Puzzled by list-appending behavior Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-27 03:52 +0000
          Re: Puzzled by list-appending behavior Chris Angelico <rosuav@gmail.com> - 2011-05-27 14:10 +1000
          RE: Puzzled by list-appending behavior "Prasad, Ramit" <ramit.prasad@jpmchase.com> - 2011-05-27 13:58 -0400
          Re: Puzzled by list-appending behavior Ethan Furman <ethan@stoneleaf.us> - 2011-05-27 11:56 -0700
  Re: Puzzled by list-appending behavior Terry Reedy <tjreedy@udel.edu> - 2011-05-26 14:44 -0400
  RE: Puzzled by list-appending behavior "Prasad, Ramit" <ramit.prasad@jpmchase.com> - 2011-05-26 15:34 -0400
  Re: Puzzled by list-appending behavior Terry Reedy <tjreedy@udel.edu> - 2011-05-26 17:36 -0400

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