On Sat, Dec 29, 2018 at 6:00 PM Sebastian Berg wrote: > > On Sat, 2018-12-29 at 17:16 +0100, Matthias Geier wrote: > > Hi Sebastian. > > > > I don't have an opinion (yet) about this matter, but I have a > > question: > > > > On Thu, Dec 27, 2018 at 12:30 AM Sebastian Berg wrote: > > > > [...] > > > > > new_arr = arr.reshape(new_shape) > > > assert np.may_share_memory(arr, new_arr) > > > > > > # Which is sometimes -- but should not be -- written as: > > > arr.shape = new_shape # unnecessary container modification > > > > [...] > > > > Why is this discouraged? > > > > Why do you call this "unnecessary container modification"? > > > > I've used this idiom in the past for exactly those cases where I > > wanted to make sure no copy is made. > > > > And if we are not supposed to assign to arr.shape, why is it allowed > > in the first place? > > Well, this may be a matter of taste, but say you have an object that > stores an array: > > class MyObject: > def __init__(self): > self.myarr = some_array > > > Now, lets say I do: > > def some_func(arr): > # Do something with the array: > arr.shape = -1 > > myobject = MyObject() > some_func(myobject) > > then myobject will suddenly have the wrong shape stored. In most cases > this is harmless, but I truly believe this is exactly why we have views > and why they are so awesome. > The content of arrays is mutable, but the array object itself should > not be muted normally.
Thanks for the example! I don't understand its point, though. Also, it's not working since MyObject doesn't have a .shape attribute. > There may be some corner cases, but a lot of the > "than why is it allowed" questions are answered with: for history > reasons. OK, that's a good point. > By the way, on error the `arr.shape = ...` code currently creates the > copy temporarily. That's interesting and it should probably be fixed. But it is not reason enough for me not to use it. I find it important that is doesn't make a copy in the success case, I don't care very much for the error case. Would you mind elaborating on the real reasons why I shouldn't use it? cheers, Matthias > > - Sebastian > > > > > > cheers, > > Matthias > > _______________________________________________ > > NumPy-Discussion mailing list > > NumPy-Discussion@python.org > > https://mail.python.org/mailman/listinfo/numpy-discussion > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@python.org > https://mail.python.org/mailman/listinfo/numpy-discussion _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion
