[Numpy-discussion] Determining the 'viewness' of an array, and flags.owndata confusion

Robert Kern robert.kern@gmail....
Tue Feb 28 17:29:12 CST 2012


On Tue, Feb 28, 2012 at 23:01, Kurt Smith <kwmsmith@gmail.com> wrote:
> For an arbitrary numpy array 'a', what does 'a.flags.owndata' indicate?
>
> I originally thought that owndata is False iff 'a' is a view.  But
> that is incorrect.
>
> Consider the following:
>
> In [119]: a = np.zeros((3,3))
>
> In [120]: a.flags.owndata  # should be True; zeros() creates and
> returns a non-view array.
> Out[120]: True
>
> In [121]: a_view1 = a[2:, :]
>
> In [122]: a_view1.flags.owndata   # expected to be False
> Out[122]: False
>
> In [123]: a_fancy1 = a[[0,1], :]
>
> In [124]: a_fancy1.flags.owndata  # expected to be True, a_fancy1 is a
> fancy-indexed array.
> Out[124]: True
>
> In [125]: a_fancy2 = a[:, [0,1]]
>
> In [126]: a_fancy2.flags.owndata # expected to be True, a_fancy2 is a
> fancy-indexed array.
> Out[126]: False
>
> So when I query an array's flags.owndata, what is it telling me?  What
> I want to know is whether an array is a view or not.  If flags.owndata
> has nothing to do with the 'viewness' of an array, how would I
> determine if an array is a view?
>
> In the previous example, a_fancy2 does not own its data, as indicated
> by 'owndata' being False.  But when I modify a_fancy2, 'a' is not
> modified, as expected, but contrary to what 'owndata' would seem to
> indicate.
>
> The numpybook's documentation of owndata could perhaps be a bit
> clearer on the subject:
>
> """
> OWNDATA (O) the array owns the memory it uses or if it borrows it from
> another object (if this is False, the base attribute retrieves a
> reference to the object this array obtained its data from)
> """
>
> >From my reading, owndata has nothing to do with the 'viewness' of an
> array.  Is this correct?  What is the intent of this flag, then?  Its
> wording could perhaps be improved: owndata is True iff (1) the array
> owns the memory it uses or (2) the array borrows it from another
> object.  The second clause seems to indicate that the array **does
> not** own its data since it is borrowing it from another object.
> However, flags.owndata will be true in this case.
>
> If I cannot use flags.owndata, what is a reliable way to determine
> whether or not an array is a view?

Your original intuition was correct. It's just that sometimes an
operation will make a copy of the input data, but then make a view on
that copy. The output object is a view, just not a view on the input
object.

-- 
Robert Kern


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