[Numpy-discussion] numarray __copy__ and __deepcopy__

Fernando Perez fperez at colorado.edu
Fri Nov 7 13:37:17 CST 2003


Tim Hochberg wrote:
> It appears that numarray.NumArray does not supply __copy__ or 
> __deepcopy__ methods and as a result copy.copy and copy.deepcopy do not 
> work correctly. It appears that adding "__copy__ = copy" to class 
> NumArray is suffcient for copy, __deepcopy__ appears to need something 
> more. Sadly, I don't have time to investigate this further right now.

As far as I understand, this isn't really necessary with Numeric/Numarray, 
because the copy() method in a sense always guarantees a 'deep copy'.  Even 
when you make assignments to slices of an array, the issue of nested 
structures which for python lists/dicts requires deepcopy() just does not 
arise.  A simple illustration:

In [1]: a=arange(10)

In [2]: b=arange(10,20)

In [3]: c=arange(20,30)

In [4]: d=zeros(30)

In [5]: d[0:10] = a

In [6]: d[10:20] = b

In [7]: d[20:30] = c

In [8]: a
Out[8]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

In [9]: b
Out[9]: array([10, 11, 12, 13, 14, 15, 16, 17, 18, 19])

In [10]: c
Out[10]: array([20, 21, 22, 23, 24, 25, 26, 27, 28, 29])

In [11]: d
Out[11]:
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
             19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29])

In [12]: b[:]=99

In [13]: b
Out[13]: array([99, 99, 99, 99, 99, 99, 99, 99, 99, 99])

In [14]: d
Out[14]:
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
             19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29])


Perhaps I'm missing some usage case, but I've always just used ARR.copy() when 
I've needed a 'full copy' of an array.  This guarantees that the returned 
array is contiguous (has .flat) and a standalone copy of the data in ARR, 
regardless of the contiguity properties of ARR.

HTH.

Cheers,

f

ps: my experience is actually from Numeric, I don't know if Numarray differs 
in its copy() behavior.





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