Augument assignment versus regular assignment

[Kirk McDonald]

nagy wrote:
> Thanks, Kirk.
> I considered the += as only a shorthand notation for the assignment
> operator.
> Since for lists + is simply a concatetation, I am not sure it x=x+[2]
> is creating a brand
> new list. Could you refer me to any documentation on this?

Yes:

http://docs.python.org/ref/augassign.html
"An augmented assignment expression like x += 1 can be rewritten as x = 
x + 1 to achieve a similar, but not exactly equal effect. In the 
augmented version, x is only evaluated once. Also, when possible, the 
actual operation is performed in-place, meaning that rather than 
creating a new object and assigning that to the target, the old object 
is modified instead."

This behavior is only logical. Consider:

 >>> x = [2]
 >>> y = x + [4]

After these operations, we have two lists: x (the list [2]) and y (the 
list [2, 4]). This is because the expression "x + [4]" creates a new 
list. We then bind this new list to the name 'y', and leave the name 'x' 
alone.

If we then say this:

 >>> x = x + [6]

We are doing much the same operation. We are creating a new list (the 
list [2, 6]), and binding it to the name 'x'. The list [2], previously 
bound to 'x', is no longer bound to anything, so Python frees it.

The augmented assignment, as I went over previously, attempts to modify 
the list object directly. Any names bound to the object (or any other 
objects that reference the object) will see the changes.

-Kirk McDonald