why cannot assign to function call

[Steven D'Aprano]

On Tue, 06 Jan 2009 13:43:01 -0800, Aaron Brady wrote:


> I think one of the ideas we have trouble communicating is that [1, 2, 3]
> and [4, 5, 6] can be the same object 

Not at the same time they can't.


> (using '[:]='), but [1, 2, 3] and [1, 2, 3] don't have to be.

I don't think this is hard to get across, if you think about objects.

You have a list. It's like a box. That box can hold items 1, 2 and 3. You 
can replace those items with 4, 5 and 6, and it's still the same box.

Likewise, you can have a box with items 1, 2 and 3 inside it, and a 
different box also with items 1, 2 and 3, and they're still different 
boxes.

The only tricky thing is that items 1, 2 and 3 can be inside two 
different boxes at the same time. There's no obvious real world analogy 
to that without the boxes being nested. This ability for objects to be in 
two places at once (or even to be inside themselves!) is one of the few 
reasons why Python's use of references in the implementation needs to be 
mentioned.



-- 
Steven