[Numpy-discussion] Re: Numeric3 PEP

Tim Hochberg tim.hochberg at cox.net
Sat Feb 19 15:10:14 CST 2005


Hi Travis,

First off, let me say that I'm encouraged to see some action towards 
unifying Numeric/Numarray the split has been somewhat dismaying. Thank 
you for your efforts in this regard.

I'd like to lobby against flatten(), r() and i(). To a large extent, 
these duplicate the functionality of flat, real and imag. And, these 
three methods are defined to sometimes return copies and sometimes 
return views. That type of interface is a recipe for errors and should 
only be used as a last resort. Fortunately in this case there are better 
alternatives.
 
Flatten() is not necessary now that flat will be faux array with a view 
to the original [I believe you are calling it an indexable iterator].  I 
would, however,  recomend that A.flat.copy() work. In that case, A.flat 
would be used when no copy was desired, and A.flat.copy() when a copy 
was desired.  I don't find the copy when discontiguous case useful 
enough to deserve it's own function and it's error prone as I'll discuss 
more below. 

r()  appears to be around just for symmetry with i() since A.r() will 
always be the same as A.real. That leaves i(). My opinion is that this 
case would be better served by returning a read-only array of zeros when 
operating on a real array. This array could even be a faux-array that 
doesn't allocate any space, although that may be a project for another day.

If it's really deemed necessary to have these functions in addition to 
their attribute brethren, I recomend that they always return copies 
rather than varying their behaviour depending on the situation. The 
problem with methods that sometimes return a copy, is that it won't be 
long before someone types:

    def foobar(a)
       flat_view = a.flatten()
       # lots of code
       flat_view[some_index] = some_new_number

This works until someone passes in a discontiguous array, at which point 
it fails mysteriously. This type of problem tends to be somewhat 
resistant to unit tests, since tests often involve only contiguous arrays.

Regards,

-tim





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