[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c (original) (raw)

Armin Rigo arigo at tunes.org
Wed Mar 29 12:45:40 CEST 2006

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 nbadd and sqconcat 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

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