why?

[Erik Max Francis]

bradh at cuneata.net wrote:

> Quoting dsavitsk <dsavitsk at e-coli.net>:

> > "Tuples, like strings, are immutable: ...
>
> That is what I thought too. But it doesn't match the observed
> behaviour.
> 
> > > >>> doing = ['a','b','c']
> > > >>> doing[1] = ('b','c')
>
> doing[1] is a string, and it shouldn't be possible to
> assign to an immutable object.

If L is a sequence, then L[i] = x is modifying the _sequence_, not the
element of the sequence:

>>> L = [1, 2, 3]
>>> L[1] = 'a'
>>> L
[1, 'a', 3]

> > > >>> doing[1][0]
> > > 'b'
>
> This is irrelevant. doing[1] should be the same as doing[1][0]
> in this particular case.

No, because at this point doing has a tuple as the second argument.  One
of the beauties of Python's interactive interpreter is that it makes it
easy to test things out.  Try it yourself:

>>> doing = ['a', 'b', 'c']
>>> doing[1] = ('b', 'c')
>>> doing
['a', ('b', 'c'), 'c']

> > > >>> doing[1][0] = 'i'
> > > Traceback (most recent call last):
> > >   File "<stdin>", line 1, in ?
> > > TypeError: object doesn't support item assignment

So the error is is that you are trying to replace the first element of
the _tuple_ with 'i'.  The problem isn't with strings, it's with tuples;
tuples are immutable and so you cannot modify them.

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