Reference or Value?

[Denis Kasak]

On Mon, Feb 23, 2009 at 5:09 AM, Steven D'Aprano
<steven at remove.this.cybersource.com.au> wrote:
> On Sun, 22 Feb 2009 13:37:27 -0300, andrew cooke wrote:
>
>> as far as i understand things, the best model is:
>>
>> 1 - everything is an object
>> 2 - everything is passed by reference
>
> Except that is wrong. If it were true, you could do this:
>
> def swap(x, y):
>    y, x = x, y
>
> a = 1
> b = 2
> swap(a, b)
> assert a == 2 and b == 1
>
>
> but you can't, it does not work. Ergo, parameter passing in Python does
> not have the same semantics as languages that use pass-by-reference, such
> as Pascal and Basic. That means that even if you can justify the claim
> "Python is pass-by-reference" by some technical argument (and I don't
> believe you can), it is misleading to make that claim without further
> qualifications.

You could, however, argue that the swap function doesn't work as
expected (e.g. from a Pascal or a C++ POV) simply because the
underlying objects aren't mutable. The objects *do* get passed by
reference; the function doesn't receive a new copy of the object and
it can examine the original object's ID. The actual culprit is not the
way objects are passed but the assignment operator, since it works by
rebinding names (as Andrew Koenig explained) and not by changing the
object itself. If the swap() function could somehow access the
underlying integer object and modify it, swapping of values would
indeed occur because the function *did* get references to the objects
passed to it.

That said, it's a rather convoluted way of explaining what happens and
calling it pass-by-object feels much better. :-)

-- 
Denis Kasak