x += ... is not the same than x = x + ... if x is mutable

Newsgroups	comp.lang.python
Date	2013-03-20 07:17 -0700
Message-ID	<b5bfb026-fea4-4aec-8185-cc8837e1559a@googlegroups.com> (permalink)
Subject	x += ... is not the same than x = x + ... if x is mutable
From	bartolome.sintes@gmail.com

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Hi,

I thought that x += ... was the same than x = x + ..., but today I have realized it is not true when operating with mutable objects.

In Python 3.3 or 2.7 IDLE (Windows) compare:
>>> a = [3]
>>> b = a
>>> a = a + [1]
>>> b
[3]

and
>>> a = [3]
>>> b = a
>>> a += [1]
>>> b
[3, 1]

Is this behaviour explained in the Python documentation? 

Thanking you in advance,
Bartolomé Sintes

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x += ... is not the same than x = x + ... if x is mutable bartolome.sintes@gmail.com - 2013-03-20 07:17 -0700
  Re: x += ... is not the same than x = x + ... if x is mutable Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-03-20 16:26 +0200
  Re: x += ... is not the same than x = x + ... if x is mutable Nobody <nobody@nowhere.com> - 2013-03-21 04:27 +0000
    Re: x += ... is not the same than x = x + ... if x is mutable "Colin J. Williams" <cjw@ncf.ca> - 2013-03-21 08:35 -0400
      Re: x += ... is not the same than x = x + ... if x is mutable Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-21 12:39 +0000

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