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Groups > comp.lang.python > #6324
| Date | 2011-05-26 16:58 +0100 |
|---|---|
| From | MRAB <python@mrabarnett.plus.com> |
| Subject | Re: Puzzled by list-appending behavior |
| References | <16c21256-48df-416a-971f-de49ca4cc981@x6g2000yqj.googlegroups.com> <BANLkTik+M=MBHSBoRmXjLYVN8rvD515CBQ@mail.gmail.com> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.2119.1306425499.9059.python-list@python.org> (permalink) |
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.
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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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