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Re: Tuples and immutability

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To python-list@python.org
From Terry Reedy <tjreedy@udel.edu>
Subject Re: Tuples and immutability
Date Wed, 12 Mar 2014 22:09:24 -0400
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On 3/12/2014 9:35 PM, Ian Kelly wrote:
> On Wed, Mar 12, 2014 at 5:20 PM, Steven D'Aprano
> <steve+comp.lang.python@pearwood.info> wrote:
>> On Tue, 11 Mar 2014 17:06:43 -0600, Ian Kelly wrote:
>>
>>> That's true but irrelevant to my point, which was to counter the
>>> assertion that mutable types can always be assumed to be able to perform
>>> operations in-place.
>>
>> "Always"? Not so fast.
>>
>> This is Python. We have freedom to abuse nearly everything, and if you
>> want to shoot yourself in the foot, you can. With the exception of a
>> handful of things which cannot be overridden (e.g. None, numeric
>> literals, syntax) you cannot strictly assume anything about anything.
>> Python does not enforce that iterators raise StopIteration when empty, or
>> that indexing beyond the boundaries of a sequence raises IndexError, or
>> that __setitem__ of a mapping sets the key and value, or that __len__
>> returns a length.
>
> Thank you; you've stated my point more succinctly than I did.
>
>> Augmented assignment is no different. The docs describe the intention of
>> the designers and the behaviour of the classes that they control, so with
>> standard built-in classes like int, str, list, tuple etc. you can safely
>> assume that mutable types will perform the operation in place and
>> immutable types won't, but with arbitrary types from some arbitrarily
>> eccentric or twisted programmer, who knows what it will do?
>
> This got me curious about how consistent the standard library is about
> this exactly, so I did some grepping.  In the standard library there
> are 5 mutable types that support concatenation that I was able to
> find: list, deque, array, bytearray, and Counter.  There are none that
> support addition, which I find interesting in that the language
> provides hooks for in-place addition but never uses them itself.
>
> All of the classes above appear to follow the rule that if you can
> concatenate an operand, you can in-place concatenate the same operand.
>   The converse however does not hold:  list.__iadd__ and
> Counter.__iadd__ are both more permissive in what types they will
> accept than their __add__ counterparts, and especially interesting to
> me is that deque implements __iadd__ but does not implement __add__ at
> all.  This last in particular seems to support the assertion that +=
> should be viewed more as a shorthand for an in-place operation, less
> as an equivalent for x = x + y.
>
>>>> l = [1,2,3]
>>>> l + (4,5,6)
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: can only concatenate list (not "tuple") to list
>>>> l += (4,5,6)
>>>> l
> [1, 2, 3, 4, 5, 6]

Like it or not, one should actually think of 'somelist += iterable' as 
equivalent to 'somelist.extend(iterable)'.  Without looking at the C 
code, I  suspect that the latter is the internal implementation. 
Collections.deque also has .extend. Collections.Counter has .update and 
that is += seems to be doing.


>>>> c = collections.Counter('mississippi')
>>>> c + collections.Counter('alabama')
> Counter({'s': 4, 'a': 4, 'i': 4, 'p': 2, 'm': 2, 'b': 1, 'l': 1})
>>>> c + dict({'a': 4, 'l': 1, 'b': 1, 'm': 1})
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: unsupported operand type(s) for +: 'Counter' and 'dict'
>>>> c += dict({'a': 4, 'l': 1, 'b': 1, 'm': 1})
>>>> c
> Counter({'s': 4, 'a': 4, 'i': 4, 'p': 2, 'm': 2, 'b': 1, 'l': 1})
>
>>>> d = collections.deque([1,2,3])
>>>> d += [4,5,6]
>>>> d
> deque([1, 2, 3, 4, 5, 6])
>>>> d + [7,8,9]
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: unsupported operand type(s) for +: 'collections.deque' and 'list'
>>>> d.__add__
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> AttributeError: 'collections.deque' object has no attribute '__add__'
>


-- 
Terry Jan Reedy

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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

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