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Puzzled by list-appending behavior

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  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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#6345

On 5/26/2011 11:58 AM, MRAB wrote:
> On 26/05/2011 06:17, Chris Rebert wrote:
>> 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.

The rule for builtin collections is that mutation methods do not return 
the collection. If there is a member of the collection to return, they 
return that. Otherwise they return None.

list/set.pop and dict.popitem are other mutation methods that have a 
(former) member to return.

The rule applies to special methods like __getitem__ (returns an item) 
and __setitem__ (returns None). Since a.append(item) is *conceptually* 
equivalent to a.__setitem(len(a), item) (I know, it raises) and 
*actually* defined as a.__setitem(len(a):len(a), item), it should not be 
surprising that all three return None.

I think the above should be better documented.
http://bugs.python.org/issue12192
has some proposals. Comments there welcome.

In another post
On 5/26/2011 4:09 AM, Chris Rebert wrote:
 > I'm just surprised that the docstring doesn't explicitly state
 > "Returns None." by this point, given that this is such a common point
 > of newbie confusion.

I never noticed. After reading the above, I added this to the proposal 
above.



-- 
Terry Jan Reedy

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