[Numpy-tickets] [NumPy] #358: Reshape makes unnecessary copies

NumPy numpy-tickets at scipy.net
Fri Oct 20 01:54:51 CDT 2006

#358: Reshape makes unnecessary copies
 Reporter:  peridot  |       Owner:  somebody
     Type:  defect   |      Status:  new     
 Priority:  normal   |   Milestone:          
Component:  Other    |     Version:          
 Severity:  normal   |    Keywords:          
 Reshape's rule for detecting when it can return a view is too
 conservative. This results in unnecessary copying of data, and it
 restricts the utility of assigning to the shape attribute (which never
 copies). Consider the following session:

 In [3]: from numpy import *

 In [4]: z = zeros((4,4))[:,1]

 In [5]: z
 Out[5]: array([ 0.,  0.,  0.,  0.])

 In [6]: r = z.reshape((2,2))

 In [7]: r[0]=1

 In [8]: z
 Out[8]: array([ 0.,  0.,  0.,  0.])

 The elements of z are not contiguous, but they are a linear array spaced
 by 4*sizeof(float); there's no reason this can't be reshaped with a view.

 Attached is a patch that recognizes all situations where a view can be
 returned. It passes all tests in numpy, and a few of my own devising
 (although it could use a more thorough and organized test suite). The new
 code is only called when a copy would have been made under the current
 rules, so it should not affect the speed of "easy" reshapes. It does
 correctly handle the easy reshapes, so the code could perhaps be
 simplified by removing some special-case detection.

Ticket URL: <http://projects.scipy.org/scipy/numpy/ticket/358>
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