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

[Kurt Smith]

On Tue, Feb 28, 2012 at 5:12 PM, Larsen, Brian A <balarsen at lanl.gov> wrote:
> Stack overflow post related to this:
> http://stackoverflow.com/questions/9164269/can-you-tell-if-an-array-is-a-view-of-another

They recommend testing "a.base".  However, in the example I posted,
"a.base" is not reliable either.

For example:

In [5]: a = zeros((3,3))

In [6]: a_fancy1 = a[[0,1], :]

In [7]: a_fancy1.base  # None

In [8]: a_fancy2 = a[:, [0,1]]

In [9]: a_fancy2.base  # Not None.  But a_fancy2 is not a view into 'a'.
Out[9]:
array([[ 0.,  0.,  0.],
       [ 0.,  0.,  0.]])

So if I test to see if 'arr.base' is not None, it may or or may not be
a view.  Is this correct?

>
>
>
>
> On Feb 28, 2012, at 4:01 PM, Kurt Smith 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?
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> --
>
> Brian A. Larsen
> ISR-1 Space Science and Applications
> Los Alamos National Laboratory
> PO Box 1663, MS-D466
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