[Numpy-discussion] optional arguments to the array constructor

Todd Miller jmiller at stsci.edu
Wed Jul 7 12:47:02 CDT 2004


On Wed, 2004-07-07 at 14:25, Philip Austin wrote:
> Todd Miller writes:
>  > On Tue, 2004-07-06 at 21:42, Philip Austin wrote:
>  > > (for numpy v1.0 on Mandrake 10 i686)
>  > 
>  > My guess is you're talking about numarray here.  Please be charitable if
>  > I'm talking out of turn... I tend to see everything as a numarray
>  > issue.
> 
> Right -- I'm still working through the boost test suite for numarray, which is
> failing a couple of tests that passed (around numarray v0.3).
> 
>  > All this looks like a documentation problem.  The numarray array()
>  > signature has been tortured by Numeric backward compatibility,  so there
>  > has been more flux in it than you would expect.  Anyway, the manual is
>  > out of date.  Here's the current signature from the code:
>  > 
>  > def array(sequence=None, typecode=None, copy=1, savespace=0,
>  >           type=None, shape=None):
>  > 
> 
> Actually, it seems to be a difference in the way that numeric and
> numarray treat the copy flag when typecode is specified.  In numeric,
> if no change in type is requested and copy=0, then the constructor
> goes ahead and produces a view:
> 
> import Numeric as nc
> test=nc.array([1,2,3],'i')
> a=nc.array(test,'i',0)
> a[0]=99
> print test
> >> [99  2  3]
> 
> but makes a copy if a cast is required:
> 
> test=nc.array([1,2,3],'i')
> a=nc.array(test,'F',0)
> a[0]=99
> print test
> >>> [1 2 3]
> 
> Looking at numarraycore.py line 305 I see that:
> 
>         if type is None and typecode is None:
>             if copy:
>                 a = sequence.copy()
>             else:
>                 a = sequence
> 
> i.e. numarray skips the check for a type match and ignores
> the copy flag, even if the type is preserved:
> 
> import numarray as ny
> test=ny.array([1,2,3],'i')
> a=ny.array(test,'i',0)
> a._data is test._data
> >>> False
> 

OK,  I think I see what you're after and agree that it's a bug.  Here's
how I'll change the behavior:

>>> import numarray
>>> a = numarray.arange(10)
>>> b = numarray.array(a, copy=0)
>>> a is b
True
>>> b = numarray.array(a, copy=1)
>>> a is b
False

One possible point of note is that array() doesn't return views for
copy=0;  neither does Numeric; both return the original sequence.

Regards,
Todd





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