References and copying

[danmcleran at yahoo.com]

> Where can I find a good explanation when does an interpreter copy the
> value, and when does it create the reference.
Any good Python book. I have Learning Python and Programming Python 2nd
edition and they are very good IMO.

> I thought I understand
> it, but I have just typed in following commands:
>
> >>> a=[[1,2],[3,4]]
> >>> b=a[1]
> >>> b=[5,6]
> >>> a
> [[1, 2], [3, 4]]
> >>> b
> [5, 6]
>
> And I don't understand it.  I thought, that b will be a reference to a,
> so changing b should change a as well.

No, you've set the name b to reference a slice of a. Slicing a list
always returns a new list.
To change a via b, do this:

a = [[1,2],[3,4]]
b = a

print a
print b

b[1] = [5,6]

print a
print b