[Numpy-discussion] copy on demand

Scott Ransom ransom at physics.mcgill.ca
Mon Jun 17 10:14:48 EDT 2002

On June 17, 2002 04:57 am, Konrad Hinsen wrote:
> > > Then the only solution I see is the current one: default behaviour is
> > > view, and when you want a copy yoy copy explicitly. The inverse is not
> > > possible, once you made a copy you can't make it behave like a view
> > > anymore.
> >
> > I don't think it is necessary to create the other object _from_
> > the default one.  You could have copy behavior be the default,
> > and if you want a view of some array you simply request one
> > explicitly with .view, .sub, or whatever.
> Let's make this explicit. Given the following four expressions,
> 1) array
> 2) array[0]
> 3) array.view
> 4) array.view[0]
> what would the types of each of these objects be according to your
> proposal? What would the indexing behaviour of those types be?
> I don't see how you can avoid having either two types or two
> different behaviours within one type.

If we assume that a slice returns a copy _always_, then I agree that #4 in 
your list above would not give a user what they would expect:

array.view[0] would give the view of a copy of array[0], _not_ a view of 
array[0] which is probably what is wanted.  I _think_ that this could be 
fixed by making view (or something similar) an option of the slice rather 
than a method of the object.  For example (assuming that a is an array):

Expression:      Returns:             Slicing Behavior:
a or a[:]        Copy of all of a     Returns a copy of the sub-array
a[0]             Copy of a[0]         Returns a copy of the sub-array
a[:,view]        View of all of a     Returns a copy of the sub-array
a[0,view]        View of a[0]         Returns a copy of the sub-array

Notice that it is possible to return a copy of a sub-array from a view since 
you have access (through a pointer) to the original array data.


Scott M. Ransom              Address:  McGill Univ. Physics Dept.
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email:  ransom at physics.mcgill.ca       Montreal, QC  Canada H3A 2T8 
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