[Numpy-discussion] Should arr.diagonal() return a copy or aview? (1.7 compatibility issue)

T J tjhnson at gmail.com
Wed May 23 19:31:52 EDT 2012

On Wed, May 23, 2012 at 4:16 PM, Kathleen M Tacina <
Kathleen.M.Tacina at nasa.gov> wrote:

> **
> On Wed, 2012-05-23 at 17:31 -0500, Nathaniel Smith wrote:
> On Wed, May 23, 2012 at 10:53 PM, Travis Oliphant <travis at continuum.io> wrote:> To be clear, I'm not opposed to the change, and it looks like we should go forward.>> In my mind it's not about developers vs. users as satisfying users is the whole point.   The purpose of NumPy is not to make its developers happy :-).   But, users also want there to *be* developers on NumPy so developer happiness is not irrelevant.>> In this case, though, there are consequences for users because of the double copy if a user wants to make their code future proof.   We are always trading off predicted user-experiences.    I hope that we all don't have the same perspective on every issue or more than likely their aren't enough voices being heard from real users.
> I'm not really worried about users who have a problem with thedouble-copy. It's a totally legitimate concern, but anyone who hasthat concern has already understood the issues well enough to be ableto take care of themselves, and decided that it's worth the effort tospecial-case this. They can check whether the returned array has .baseset to tell whether it's an array or a view, use a temporary hack tocheck for the secret warning flag in arr.flags.num, check the numpyversion, all sorts of things to get them through the one version wherethis matters. The suggestion in the docs to make a copy is not exactlybinding :-).
> -- Nathaniel
> As a "real user", if I care about whether an array arr2 is a copy or a
> view, I usually either check arr2.flags.owndata or append copy() to the
> statement that created arr2, e.g., arr2 = arr.diagonal().copy().
> Numpy rules on views vs. copies sometimes require a bit of thought, and so
> I'll frequently just check the flags or make a copy instead of thinking.
> (More foolproof :).)

 It seems that there are a number of ways to check if an array is a view.
 Do we have a preferred way in the API that is guaranteed to stay
available?  Or are all of the various methods "here to stay"?
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