Thank you all for your answers. Kurt and I were training new developers on numpy and telling them that - fancy indexing creates a copy - owndata was a good way to know if an array is a view or a copy. It turns out that these were both correct statements but we didn't think that the fancy indexing would sometimes return a view after making a copy (BTW is that necessary? It sounds to me like it is complicating things for no real benefit.)
So a more precise question then is "is a_fancy a view on a's data or does it copy the data and its data is independent?" and the answer is probably: since "a_fancy1.base is a" is false then the 2 array's data are independent from each other. Do I have it right now? Jonathan On Tue, Feb 28, 2012 at 5:31 PM, Nathaniel Smith <n...@pobox.com> wrote: > On Tue, Feb 28, 2012 at 11:01 PM, Kurt Smith <kwmsm...@gmail.com> wrote: > > For an arbitrary numpy array 'a', what does 'a.flags.owndata' indicate? > > I think what it really indicates is whether a's destructor should call > free() on a's data pointer. > > > 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. > > It looks like the fancy indexing code in this case actually creates a > new array, and then instead of returning it directly, returns a > (reshaped) view on this new array. If you look at a_fancy2.base, > you'll see this new array. > > So: a_fancy2 *is* a view... it's just not a view of 'a'. It's a view > of this other array. > > > If I cannot use flags.owndata, what is a reliable way to determine > > whether or not an array is a view? > > owndata *is* a reliable way to determine whether or not an array is a > view; it just turns out that this is not a very useful question to > ask. > > What are you actually trying to do? There's probably another way to > accomplish it. > > -- Nathaniel > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > -- Jonathan Rocher, PhD Scientific software developer Enthought, Inc. jroc...@enthought.com 1-512-536-1057 http://www.enthought.com
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