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Re: Tuples and immutability

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From Joshua Landau <joshua@landau.ws>
Date Fri, 28 Feb 2014 14:41:20 +0000
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Subject Re: Tuples and immutability
To Chris Angelico <rosuav@gmail.com>
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Xref csiph.com comp.lang.python:67225

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On 27 February 2014 16:14, Chris Angelico <rosuav@gmail.com> wrote:
> On Fri, Feb 28, 2014 at 3:01 AM, Eric Jacoboni <eric.jacoboni@gmail.com> wrote:
>>
>>>>> a_tuple = ("spam", [10, 30], "eggs")
>>>>> a_tuple[1] += [20]
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: 'tuple' object does not support item assignment
>>
>> Ok... I accept this message as += is a reassignment of a_tuple[1] and a
>> tuple is immutable...
>>
>> But, then, why a_tuple is still modified?
>
> This is a common confusion.
>
> The += operator does two things. First, it asks the target to please
> do an in-place addition of the other operand. Then, it takes whatever
> result the target gives back, and assigns it back into the target. So
> with a list, it goes like this:
>
>>>> foo = [10, 30]
>>>> foo.__iadd__([20])
> [10, 30, 20]
>>>> foo = _

Would it be better to add a check here, such that if this gets raised
to the top-level it includes a warning ("Addition was inplace;
variable probably mutated despite assignment failure")?

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Thread

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 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 "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

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