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

[Ralf Gommers]

On Thu, Dec 27, 2018 at 3:27 PM Juan Nunez-Iglesias <jni.soma at gmail.com>
wrote:

> On Fri, Dec 28, 2018, at 3:40 AM, Sebastian Berg wrote:
>
> > It’s consistent with np.newaxis in spelling (modulo the _)
> > If mistyped, it can be caught easily by IDEs.
> > It’s typeable with mypy, unlike constant string literals which
> > currently aren’t
> > If code written against new numpy is run on old numpy, it will error
> > rather than doing the wrong thing
>
> I am not sure I think that the above are too strong arguments, since it
> is not what is typically done for keywords (nobody suggests np.CLIP
> instead of "clip"). Unless you feel this is different because it is a
> mix of strings and bools here?
>
>
> :+1: to Eric's list. I don't think it's different because of the mix, I
> think it's different because deprecating things is painful.
>

Technically there's nothing we are deprecating. If anything, the one not
super uncommon pattern will be that people use 0/1 instead of False/True,
which works as expected now and will start raising in case we'd go with
np.never_copy

But now that we have good typing in Python, I think of string literals as
> an anti-pattern going forward.
>

I don't quite get that, it'll still be the most Pythonic thing to do in
many API design scenarios. Adding a new class instance with unusual
behavior like raising on bool() is not even a pattern, it would just be an
oddity.


> But, for me, the biggest reason is the silent different behaviour in old
> vs new NumPy. As a package maintainer this is just a nightmare.
>

That is the main downside indeed in this particular case.


> I have a lot of sympathy for the ugliness argument, but I don't think
> `np.never_copy` (or e.g. `np.modes.never`) is that much worse than a string.
>

The point is not that `.reshape(..., copy=np.never_copy)` is uglier (it is,
but indeed just a little), but that we're adding a new object to the numpy
namespace that's without precedent. If we'd do that regularly the API would
really blow up.

np.newaxis is not relevant here - it's a simple alias for None, is just
there for code readability, and is much more widely applicable than
np.never_copy would be.

Cheers,
Ralf
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