On 22 March 2010 14:42, Pauli Virtanen <pav@iki.fi> wrote:
la, 2010-03-20 kello 17:36 -0400, Anne Archibald kirjoitti:
I was in on that discussion. My recollection of the conclusion was that on the one hand they're useful, carefully applied, while on the other hand they're very difficult to reliably detect (since you don't want to forbid operations on non-overlapping slices of the same array).
I think one alternative brought up was
copy if unsure whether the slices overlap
which would make
A[whatever] = A[whatever2]
be always identical in functionality to
A[whatever] = A[whatever2].copy()
which is how things should work. This would permit optimizing simple cases (at least 1D), and avoids running into NP-completeness (for numpy, the exponential growth is however limited by NPY_MAXDIMS which is 64, IIRC).
It can produce surprise copies, but I could certainly live with this. Or maybe a slight modification: "always produces values equivalent to using a copy", to allow handling the common A[:-1]=A[1:] without a copy. Of course, we'd have to wait for someone to implement it...
This would be a change in semantics, but in a very obscure corner that hopefully nobody relies on.
It would certainly be nice to replace unspecified behaviour by specified behaviour if it can be done with minimal cost. And I think it could be, with some careful programming. Anne