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

[Travis E. Oliphant]

Greg Ewing wrote:
> Travis E. Oliphant wrote:
> 
>> I think this can be fixed easily by first checking the sequence slot for 
>> a sq_concat function before calling PyNumber_InPlaceAdd.
> 
> However, if this *is* fixed, it looks like it's going to
> break NumPy, in the sense that it will no longer be able
> to force an arithmetic interpretation for "+" when adding
> a sequence to an array.

Well, it won't break any "documented" behavior of NumPy which does not 
use the sq_concat (or sq_concat_inplace) slot at all.

I can't imaging anybody relying on an in-place operations to return a 
"different" object, but we could make the change and run all the 
NumPy/SciPy tests to see what happens.

But, you are right that

w = [1,2,3,4]

w += array([1,2,3,4])

would no longer make w equal to array([2,4,6,8]).  But, this seems like 
a very good thing to me, as such behavior is actually quite hard to 
explain except as "its a bug"

-Travis