Augument assignment versus regular assignment

[nagy]

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?
Thanks,
Nagy
Kirk McDonald wrote:
> 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
> >