Why does changing 1 list affect the other?

[Ben Finney]

On Fri, 07 Nov 2003 03:00:29 GMT, Cy Edmunds wrote:
> "Ben Finney" <bignose-hates-spam at and-benfinney-does-too.id.au> wrote:
>> What you probably want os to take a *copy* of the list object, and
>> bind 'secondlist' to that new object.  This occurs automatically for
>> some types (e.g.  scalars) but not lists or dicts or other structured
>> types.
> 
> I used to think this too: different types work differently. Now I
> think that the only difference is that some types are immutable.

Your examples aren't showing this:

> Consider:
> 
>>>> x = 5
>>>> y = x
>>>> print x, y
> 5 5
>>>> x = 4
>>>> print x, y
> 4 5

Creating a new integer object, 4, and binding 'x' to that.

>>>> x = [5]
>>>> y = x
>>>> print x, y
> [5] [5]
>>>> x = [4]
>>>> print x, y
> [4] [5]

Creating a new list object, [4], and binding 'x' to that.

> Scalars and lists work the same! And yet:

For binding to a new object, sure.  The difference was the conceptual
"modify" operation:

    >>> x = [ 1, 2, 3 ]
    >>> y = x
    >>> print x, y
    [1, 2, 3] [1, 2, 3]
    >>> x[0] = 55
    >>> print x, y
    [55, 2, 3] [55, 2, 3]

There's no equivalent for integer objects, because they're not mutable.
I think we're in agreement, though I may have been remiss in failing to
distinguish the extra operations available to mutable types.

-- 
 \             "A good politician is quite as unthinkable as an honest |
  `\                                    burglar."  -- Henry L. Mencken |
_o__)                                                                  |
Ben Finney <http://bignose.squidly.org/>