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Re: Tuples and immutability

References (3 earlier) <mailman.7473.1393598638.18130.python-list@python.org> <XnsA2E95FA1E1EB6duncanbooth@127.0.0.1> <bnvctpF5vanU1@mid.individual.net> <mailman.7920.1394252278.18130.python-list@python.org> <bo1rjpFll5mU1@mid.individual.net>
From Ian Kelly <ian.g.kelly@gmail.com>
Date 2014-03-08 19:39 -0700
Subject Re: Tuples and immutability
Newsgroups comp.lang.python
Message-ID <mailman.7944.1394332838.18130.python-list@python.org> (permalink)

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On Sat, Mar 8, 2014 at 5:40 PM, Gregory Ewing
<greg.ewing@canterbury.ac.nz> wrote:
> Ian Kelly wrote:
>>
>> class LessThanFilter:
>>
>>     def __init__(self, the_list):
>>         self._the_list = the_list
>>
>>     def __getitem__(self, bound):
>>         return [x for x in self._the_list if x < bound]
>>
>>
>> filter = LessThanFilter([10, 20, 30, 40, 50])
>> filter[25] += [15, 17, 23]
>>
>> Should that last line not raise an exception?
>
>
> In this case it will fail to catch what is probably an error,
> but you can't expect the language to find all your bugs for
> you. If you wrote the same bug this way:
>
>    filter[25].extend([15, 17, 23])
>
> it wouldn't be caught either.
>
> What's happening is that we're trying to use the syntax
> a += b to mean two different things:
>
> 1) Shorthand for a = a + b
>
> 2) A way of expressing an in-place modification, such
>    as a.extend(b)
>
> Case (2) is not really an assignment at all, so arguably
> it shouldn't require the LHS to support assignment.

In my view the second one is wrong.  a += b should be understood as
being equivalent to a = a + b, but with the *possible* and by no means
guaranteed optimization that the operation may be performed in-place.

In fact, if you read the documentation for lists, you may notice that
while they clearly cover the + operator and the extend method, they do
not explicitly document the list class's += operator.  So although I'm
not entirely sure whether it is intentional or not, and I would be
quite surprised if some implementation were actually to differ on this
point, the language does *not* from what I can see guarantee that the
+= operator on lists is equivalent to calling .extend.

That having been said, code that uses += and relies on the operation
to be performed in-place should be considered buggy.  If you need the
operation to be performed in-place, then use in-place methods like
list.extend.  If you need the operation not to be performed in-place,
then use a = a + b.  If you're ambivalent on the in-place issue and
just want to write polymorphic code, that's when you should consider
using +=.

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Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-02-27 17:01 +0100
  Re: Tuples and immutability Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-27 10:13 -0600
    Re: Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-02-27 17:27 +0100
      Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-02-28 03:33 +1100
      Re: Tuples and immutability Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-27 10:47 -0600
      Re: Tuples and immutability Nick Timkovich <prometheus235@gmail.com> - 2014-02-27 15:47 -0600
      Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-02-28 08:52 +1100
      Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-27 15:18 -0700
        Re: Tuples and immutability Rick Johnson <rantingrickjohnson@gmail.com> - 2014-03-11 19:01 -0700
          Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-12 13:10 +1100
          Re: Tuples and immutability Terry Reedy <tjreedy@udel.edu> - 2014-03-11 23:25 -0400
            Re: Tuples and immutability Steven D'Aprano <steve@pearwood.info> - 2014-03-12 06:28 +0000
              Re: Tuples and immutability Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-12 10:39 +0100
              Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-12 03:40 -0600
              Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-12 03:51 -0600
              Re: Tuples and immutability Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-12 12:21 +0100
          Re: Tuples and immutability Ethan Furman <ethan@stoneleaf.us> - 2014-03-11 23:32 -0700
          Re: Tuples and immutability Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-03-12 10:20 +0000
      Re: Tuples and immutability Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-03-01 18:55 +0000
  Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-02-28 03:14 +1100
  Re: Tuples and immutability Marko Rauhamaa <marko@pacujo.net> - 2014-02-27 18:19 +0200
    Re: Tuples and immutability John O'Hagan <research@johnohagan.com> - 2014-02-28 16:17 +1100
      Re: Tuples and immutability Marko Rauhamaa <marko@pacujo.net> - 2014-02-28 09:54 +0200
  Re: Tuples and immutability Joshua Landau <joshua@landau.ws> - 2014-02-28 14:41 +0000
  Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-01 01:43 +1100
    Re: Tuples and immutability Duncan Booth <duncan.booth@invalid.invalid> - 2014-03-07 09:33 +0000
      Re: Tuples and immutability Ben Finney <ben+python@benfinney.id.au> - 2014-03-07 22:04 +1100
      Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-07 22:11 +1100
      Re: Tuples and immutability Peter Otten <__peter__@web.de> - 2014-03-07 12:38 +0100
      Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-07 22:45 +1100
      Re: Tuples and immutability Alister <alister.ware@ntlworld.com> - 2014-03-07 11:51 +0000
        Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-07 11:23 -0700
      Re: Tuples and immutability Roy Smith <roy@panix.com> - 2014-03-07 08:37 -0500
      Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-08 15:17 +1300
        Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-07 21:17 -0700
          Balanced trees (was: Re: Tuples and immutability) Marko Rauhamaa <marko@pacujo.net> - 2014-03-08 10:34 +0200
            Re: Balanced trees (was: Re: Tuples and immutability) Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-08 04:03 -0700
              Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-08 13:26 +0200
            Re: Balanced trees (was: Re: Tuples and immutability) Dan Stromberg <drsalists@gmail.com> - 2014-03-08 11:58 -0800
            Re: Balanced trees Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-08 20:37 +0000
              Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-08 23:21 +0200
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-08 17:22 -0500
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-09 11:17 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-08 17:31 -0800
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-09 11:27 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-09 14:20 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-09 23:32 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-09 14:37 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-09 23:43 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-09 15:08 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-10 00:24 +0200
                Re: Balanced trees Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-09 18:04 -0600
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-10 03:24 +0000
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-10 03:24 +0000
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-10 08:16 +0200
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-10 08:53 +0000
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-10 11:41 +0200
                Re: Balanced trees Ned Batchelder <ned@nedbatchelder.com> - 2014-03-10 06:57 -0400
                Re: Balanced trees Rustom Mody <rustompmody@gmail.com> - 2014-03-10 09:01 -0700
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-11 02:02 +0000
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-10 22:20 -0400
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 13:29 +1100
                Re: Balanced trees Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-10 09:51 +0000
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-10 09:59 -0400
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 03:20 +1100
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 03:24 +1100
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-10 19:08 +0200
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 04:17 +1100
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-10 19:34 +0200
                Re: Balanced trees Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-13 12:40 -0600
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-13 23:57 +0000
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-13 20:12 -0700
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-14 05:02 +0000
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-10 19:24 -0400
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 10:27 +1100
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-11 01:26 +0000
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 12:45 +1100
                Re: Balanced trees Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-10 21:38 -0600
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 15:28 +1100
                Re: Balanced trees "Rhodri James" <rhodri@wildebst.org.uk> - 2014-03-12 00:57 +0000
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-11 12:12 +0200
                Re: Balanced trees alex23 <wuwei23@gmail.com> - 2014-03-12 10:13 +1000
                Re: Balanced trees Alister <alister.ware@ntlworld.com> - 2014-03-11 09:25 +0000
                Re: Balanced trees Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-12 10:08 +0100
                Re: Balanced trees Tim Chase <python.list@tim.thechases.com> - 2014-03-10 11:33 -0500
                Re: Balanced trees Chris Angelico <rosuav@gmail.com> - 2014-03-11 03:39 +1100
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-10 18:05 -0700
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-10 22:13 -0400
                Re: Balanced trees Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-03-10 19:57 -0400
            Re: Balanced trees Joshua Landau <joshua@landau.ws> - 2014-03-15 01:13 +0000
              Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-18 00:05 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-18 12:26 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-18 22:55 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-18 14:45 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-19 00:03 +0200
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-18 15:21 -0700
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-19 01:11 +0200
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-19 01:15 +0000
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-19 10:49 +0200
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-19 13:42 +0000
                Re: Balanced trees Marko Rauhamaa <marko@pacujo.net> - 2014-03-19 15:54 +0200
                Re: Balanced trees Roy Smith <roy@panix.com> - 2014-03-19 10:06 -0400
                Re: Balanced trees Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-19 01:15 +0000
                Re: Balanced trees Ethan Furman <ethan@stoneleaf.us> - 2014-03-19 08:15 -0700
                Re: Balanced trees "Rhodri James" <rhodri@wildebst.org.uk> - 2014-03-20 02:16 +0000
                Re: Balanced trees Dan Stromberg <drsalists@gmail.com> - 2014-03-19 19:34 -0700
                Re: Balanced trees Chris Kaynor <ckaynor@zindagigames.com> - 2014-03-18 18:02 -0700
                Re: Balanced trees Daniel Stutzbach <stutzbach@google.com> - 2014-03-18 13:18 -0700
            blist in standard library  (was Re: Balanced trees) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-15 12:31 +0000
            Re: Balanced trees Daniel Stutzbach <stutzbach@google.com> - 2014-03-17 14:16 -0700
            Re: Balanced trees Joshua Landau <joshua@landau.ws> - 2014-03-18 00:08 +0000
            Re: Balanced trees Daniel Stutzbach <stutzbach@google.com> - 2014-03-17 18:01 -0700
            Re: Balanced trees Joshua Landau <joshua@landau.ws> - 2014-03-18 07:46 +0000
          Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-09 13:40 +1300
            Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-08 19:39 -0700
              Re: Tuples and immutability Marko Rauhamaa <marko@pacujo.net> - 2014-03-09 11:35 +0200
              Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-10 11:03 +1300
                Re: Tuples and immutability Terry Reedy <tjreedy@udel.edu> - 2014-03-09 19:00 -0400
                Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-09 17:42 -0600
                Re: Tuples and immutability Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-10 02:37 +0000
                Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-10 02:35 -0600
                Re: Tuples and immutability Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-10 09:13 +0000
                Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-11 18:15 +1300
                Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-11 18:03 +1300
                Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-11 04:39 -0600
                Re: Tuples and immutability Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-11 16:46 +0000
                Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-12 10:23 +1300
                Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-11 17:06 -0600
                Re: Tuples and immutability Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-12 23:20 +0000
                Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-12 19:35 -0600
                Re: Tuples and immutability Terry Reedy <tjreedy@udel.edu> - 2014-03-12 22:09 -0400
          Re: Tuples and immutability Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-09 13:45 +1300
            Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-08 19:55 -0700
  Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-02-28 16:22 -0800
    Re: Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-03-01 02:27 +0100
      Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-02-28 20:45 -0800
        Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-28 22:34 -0700
          Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-02-28 21:50 -0800
            Re: Tuples and immutability Ned Batchelder <ned@nedbatchelder.com> - 2014-03-01 12:56 -0500
      Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-28 22:26 -0700
    Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-01 12:39 +1100
    Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-28 22:16 -0700
      Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-02-28 22:16 -0800
        Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-01 17:29 +1100
        Re: Tuples and immutability Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-01 14:54 +0000
          Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-03-01 13:01 -0800
      Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-02-28 22:25 -0800
        Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-01 12:45 -0700
      Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-03-01 13:21 -0800
        Re: Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-03-02 03:04 +0100
          Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-03-01 18:28 -0800
          Re: Tuples and immutability Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-02 05:32 -0700
            Re: Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-03-02 14:38 +0100
              Re: Tuples and immutability Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-02 14:05 +0000
                Re: Tuples and immutability Eric Jacoboni <eric.jacoboni@gmail.com> - 2014-03-02 15:17 +0100
                Re: Tuples and immutability "albert visser" <albert.visser@gmail.com> - 2014-03-02 15:37 +0100
        Re: Tuples and immutability "Mark H. Harris" <harrismh777@gmail.com> - 2014-03-01 18:15 -0800
  Re: Tuples and immutability Joshua Landau <joshua@landau.ws> - 2014-03-09 17:54 +0000
  Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-10 05:13 +1100
  Re: Tuples and immutability Joshua Landau <joshua@landau.ws> - 2014-03-09 19:57 +0000
  Re: Tuples and immutability Chris Angelico <rosuav@gmail.com> - 2014-03-10 07:06 +1100

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