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Date: Thu, 2 Feb 2012 14:18:12 +1100
From: John O'Hagan <research@johnohagan.com>
To: python-list@python.org
Subject: Re: copy on write
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:19765

On Fri, 13 Jan 2012 10:40:47 -0800
Ethan Furman <ethan@stoneleaf.us> wrote:

> Steven D'Aprano wrote:
> > Normally this is harmless, but there is one interesting little
> > glitch you can get:
> > 
> >>>> t = ('a', [23])
> >>>> t[1] += [42]
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> > TypeError: 'tuple' object does not support item assignment
> >>>> t
> > ('a', [23, 42])

IMHO, this is worthy of bug-hood: shouldn't we be able to conclude from the TypeError that the assignment failed?

> There is one other glitch, and possibly my only complaint:
> 
> --> a = [1, 2, 3]
> --> b = 'hello, world'
> --> a = a + b
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: can only concatenate list (not "str") to list
> --> a += b
> --> a
> [1, 2, 3, 'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']
> 
> IMO, either both + and += should succeed, or both should fail.
> 
> ~Ethan~


This also happens for tuples, sets, generators and range objects (probably any iterable), AFAIK only when the left operand is a list. Do lists get special treatment in terms of implicitly converting the right-hand operand?

The behaviour of the "in-place" operator could be more consistent across types:

>>> a=[1,2]
>>> a+=(3,4)
>>> a
[1, 2, 3, 4]
>>> a=(1,2)
>>> a+=(3,4)
>>> a
(1, 2, 3, 4)
>>> a=(1,2)
>>> a+=[3,4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate tuple (not "list") to tuple


John