[Numpy-discussion] Add guaranteed no-copy to array creation and reshape?

[Matthias Geier]

On Sat, Dec 29, 2018 at 6:00 PM Sebastian Berg wrote:
>
> On Sat, 2018-12-29 at 17:16 +0100, Matthias Geier wrote:
> > Hi Sebastian.
> >
> > I don't have an opinion (yet) about this matter, but I have a
> > question:
> >
> > On Thu, Dec 27, 2018 at 12:30 AM Sebastian Berg wrote:
> >
> > [...]
> >
> > > new_arr = arr.reshape(new_shape)
> > > assert np.may_share_memory(arr, new_arr)
> > >
> > > # Which is sometimes -- but should not be -- written as:
> > > arr.shape = new_shape  # unnecessary container modification
> >
> > [...]
> >
> > Why is this discouraged?
> >
> > Why do you call this "unnecessary container modification"?
> >
> > I've used this idiom in the past for exactly those cases where I
> > wanted to make sure no copy is made.
> >
> > And if we are not supposed to assign to arr.shape, why is it allowed
> > in the first place?
>
> Well, this may be a matter of taste, but say you have an object that
> stores an array:
>
> class MyObject:
>     def __init__(self):
>         self.myarr = some_array
>
>
> Now, lets say I do:
>
> def some_func(arr):
>     # Do something with the array:
>     arr.shape = -1
>
> myobject = MyObject()
> some_func(myobject)
>
> then myobject will suddenly have the wrong shape stored. In most cases
> this is harmless, but I truly believe this is exactly why we have views
> and why they are so awesome.
> The content of arrays is mutable, but the array object itself should
> not be muted normally.

Thanks for the example! I don't understand its point, though.
Also, it's not working since MyObject doesn't have a .shape attribute.

> There may be some corner cases, but a lot of the
> "than why is it allowed" questions are answered with: for history
> reasons.

OK, that's a good point.

> By the way, on error the `arr.shape = ...` code currently creates the
> copy temporarily.

That's interesting and it should probably be fixed.

But it is not reason enough for me not to use it.
I find it important that is doesn't make a copy in the success case, I
don't care very much for the error case.

Would you mind elaborating on the real reasons why I shouldn't use it?

cheers,
Matthias

>
> - Sebastian
>
>
> >
> > cheers,
> > Matthias
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