On Mi, 2015-02-04 at 07:22 +0000, David Kershaw wrote:

The numpy reference manual, array objects/indexing/advance indexing, says: Advanced indexing always returns a copy of the data (contrast with basic slicing that returns a view).

If I run the following code: import numpy as np d=range[2] x=np.arange(36).reshape(3,2,3,2) y=x[:,d,:,d] y+=1 print x x[:,d,:,d]+=1 print x then the first print x shows that x is unchanged as it should be since y was a copy, not a view, but the second print x shows that all the elements of x with 1st index = 3rd index are now 1 bigger. Why did the left side of x[:,d,:,d]+=1 act like a view and not a copy?

Python has a mechanism both for getting an item and for setting an item. The latter will end up doing this (python already does this for us): x[:,d,:,d] = x[:,d,:,d] + 1 so there is an item assignment going on (__setitem__ not __getitem__) - Sebastian

Thanks, David

_______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion