On Thu, May 24, 2012 at 4:10 PM, Nathaniel Smith <n...@pobox.com> wrote: > On Thu, May 24, 2012 at 3:56 PM, Jonathan T. Niehof <jnie...@lanl.gov> wrote: >> On 05/23/2012 05:31 PM, T J wrote: >> >>> 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"? >> >> We've settled on checking array.base, which I think was the outcome of a >> stackoverflow thread that I can't dig up. (I'll check with the guy who >> wrote the code.) > > The problem is that "is a view" isn't a very meaningful concept... > checking .base will tell you whether writes to an array are likely to > affect some object that existed before that array was created. But it > doesn't tell you whether writes to that array can affect any > *particular* other object (at least without a fair amount of groveling > around the innards of both objects), and it can happen that an object > has base == None yet writes to it will affect another object, and it > can happen that an object has base != None and yet writes to it won't > affect any object that was ever accessible to your code. AFAICT it's > really these other questions that one would like to answer, and > checking .base won't answer them.
numpy.may_share_memory() gets closer, but it can be defeated by certain striding patterns. At least, it is conservative and reports false positives but not false negatives. Implementing numpy.does_share_memory() correctly involves some number theory and hairy edge cases. (Hmm, now that I think about it, the edge cases are when the strides are 0 or negative. 0-stride axes can simply be removed, and I think we should be able to work back to a first item and flip the sign on the negative strides. The typical positive-stride solution can be found in an open source C++ global array code, IIRC. Double-hmmm...) -- Robert Kern _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion