[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c

[Armin Rigo]

Hi Greg,

On Wed, Mar 29, 2006 at 12:38:55PM +1200, Greg Ewing wrote:
> I'm really thinking more about the non-inplace operators.
> If nb_add and sq_concat are collapsed into a single slot,
> it seems to me that if you do
> 
>    a = [1, 2, 3]
>    b = array([4, 5, 6])
>    c = a + b
> 
> then a will be asked "Please add yourself to b", and a
> will say "Okay, I know how to do that!" and promptly
> concatenate itself with b.

No: there is a difference between + and += for lists.  You can only
concatenate exactly a list to a list.  Indeed:

   >>> [].__add__((2, 3))
   TypeError: can only concatenate list (not "tuple") to list

By contrast, list += is like extend() and accepts any iterable.
So if we provide a complete fix, [].__add__(x) will be modified to
return NotImplemented instead of raising TypeError if x is not a list,
and then [1,2,3]+array([4,5,6]) will fall back to array.__radd__() as
before.

I'll try harder to see if there is a reasonable example whose behavior
would change...


A bientot,

Armin