Augument assignment versus regular assignment

[Kirk McDonald]

nagy wrote:
> I do the following. First create lists x,y,z. Then add an element to x
> using the augumented assignment operator. This causes all the other
> lists to be changed also.
> But if I use the assignment x=x+[4] instead of using the augumented
> assignment, the y and z lists do not change.
> Why is that?
> This does not happen when I work with integer data type for example, as
> shown below.
> 
> Thanks for your help
> Nagy
> 
> 
>>>>x=y=z=[]

In this example, the '[]' creates a new list object. x, y, and z are all 
set to reference that object.

>>>>x+=[2]

This does an "in-place" operation on that list, modifying (or 
"mutating") the object directly.

>>>>x
> 

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

This creates a new list that is the concatenation of the list created 
above (the list [2]) with a new list (the list [4]). This brand new list 
is bound to the name 'x'. The names 'y' and 'z' are left unchanged. That 
is, they still point to the original list.

>>>>
>>>>x
> 
> [2, 4]
> 
>>>>y
> 
> [2]
> 
>>>>z
> 
> [2]
> 
>>>>a=b=4

This binds the names 'a' and 'b' to the integer object 4.

>>>>b
> 
> 4
> 
>>>>a+=2

This attempts to mutate the integer object 4, by adding 2 to it. 
However, numbers in Python are immutable, and so the in-place operation 
fails. Thus, it creates a new integer object equal to 6 (actually, 
CPython keeps a cache of certain smaller integer objects and reuses 
them, but this does not matter in practice). This new integer object is 
bound to the name 'a'. The name 'b' remains bound to the original 4 object.

>>>>a
> 
> 6
> 
>>>>b
> 
> 4
>