On Sun, 2018-12-30 at 16:03 +0100, Matthias Geier wrote: > 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. >
The example should have called `some_func(myobject.arr)`. The thing is that if you have more references to the same array around, you change all their shapes. And if those other references are there for a reason, that is not what you want. That does not matter much in most cases, but it could change the shape of an array in a completely different place then intended. Creating a new view is cheap, so I think such things should be avoided. I admit, most code will effectively do: arr = input_arr[...] # create a new view arr.shape = ... so that there is no danger. But conceptually, I do not think there should be a danger of magically changing the shape of a stored array in a different part of the code. Does that make some sense? Maybe shorter example: arr = np.arange(10) arr2 = arr arr2.shape = (5, 2) print(arr.shape) # also (5, 2) so the arr container (shape, dtype) is changed/muted. I think we expect that for content here, but not for the shape. - Sebastian > > 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 >
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