[Numpy-discussion] augmented assignment and in-place operations

[Alan G Isaac]

On 10/29/2008 3:43 PM Robert Kern wrote:
> The defining characteristic is 
> that "x <op>= y" should be equivalent to "x = x <op> y" except 
> possibly for *optional* in-place semantics. 

This gets at a bit of the Language Reference that I've
never understood.

         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.

What does that mean?  I assume "when possible" means in part
"when mutable", ruling out e.g. Python integers or floats.
But the rest does not really seem part of the language, but
rather seems to be normative?  That is, I could define
__iadd__ anyway I want.  Is the above saying that I "should"
define __iadd__ to do an in-place operation "if possible".
If so, why is such a normative statement part of the
language reference?  Or is it a statement about the language
that I'm just not getting?

Confusedly,
Alan