[Cython] Cython array type: Summary, introducing CEP 518

[Dag Sverre Seljebotn]

Dag Sverre Seljebotn wrote:
> Stefan Behnel wrote:
>> Dag Sverre Seljebotn wrote:
>>>> # Some ways of multiplying all elements with 2
>>>> x *= 2
>>>> x[...] *= 2
>>>> x[:,:] *= 2
>>>> x += x
>>>> x[...] += x
>>> OK I'll make an exception here -- I'm willing to discuss whether we 
>>> should depart from NumPy semantics here and let
>>>
>>> x2 = x
>>> x *= 2
>>>
>>> allocate new memory, so that x2 is not modified, being consistent with a 
>>> direct transformation to "x = x * 2". One can always write
>>>
>>> x[...] *= 2
>>>
>>> if one wishes to modify original memory.
>> What's wrong with
>>
>> 	x = 2 * x2
>>
>> for doing a copy ?
>>
>> x *= 2
>>
>> pretty clearly states that I want to modify x in place.
> 
> The problem is that if you do
> 
> y = x
> x *= 2
> 
> then in current NumPy, y and x will still point to the same memory and 
> reference the same values (in fact, be the exact same view object).
> 
> Usual Python semantics seems to imply that what NumPy *should* have done 
> is let the latter line mean "x = x * 2", where a new array is allocated 
> and the value of x*2 copied into the new array, so that x and y points 
> to different memory after the operation.

I stand corrected:

In [13]: a = [1,2,3]

In [14]: b = a

In [15]: a += [1,2,3]

In [16]: b
Out[16]: [1, 2, 3, 1, 2, 3]

OK, so this seems like a non-issue.


-- 
Dag Sverre