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From: Zachary Ware <zachary.ware+pylist@gmail.com>
Date: Thu, 27 Feb 2014 10:13:30 -0600
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Subject: Re: Tuples and immutability
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On Thu, Feb 27, 2014 at 10:01 AM, Eric Jacoboni <eric.jacoboni@gmail.com> wrote:
> Hi,
>
> I'm using Python 3.3 and i have a problem for which i've still not found
> any reasonable explanation...
>
>>>> 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?
>
>>>> a_tuple
> ('spam', [10, 30, 20], 'eggs')
>
> I get a TypeError for an illegal operation, but this operation is still
> completed?

You're not the first person to have this question :)

http://docs.python.org/3/faq/programming.html#why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works

-- 
Zach