[Numpy-discussion] Ransom Proposals

Christopher Barker Chris.Barker at noaa.gov
Tue Mar 28 10:45:05 CST 2006

I want to start by saying a am +1 on everything Tim is advocating, just 
so you all know it's not just him. I'm not writing any code, however.

A few comments:

Travis Oliphant wrote:

> It would seem that most of Tim's complaints are directly against 
> polymorphism.

I don't think so. I think Tim (and I) just want it to be really clear 
what the polymorphic functions are and what aren't. Also, for 
polymorphism to work, there always has to be some definition of what 
"similar" objects are: in static languages, this means they are derived 
from the same class. In duck-typed languages, it means they share the 
same behavior, so all we're doing here is deciding exactly how much 
behavior they need to share.

 >  But, then Python has
> this same problem, because
> l += a
> doesn't do 'in-place' for the list, but does do inplace if 'l' were an 
> array.

Yes, it does do it for the list, but differently. However, maybe a tuple 
is a better example:

 >>> a = (1,2,3)
 >>> id(a)
 >>> a += (3,4,5)
 >>> a
(1, 2, 3, 3, 4, 5)
 >>> id(a)

Whereas with numpy:

 >>> import numpy as N
 >>> a = N.array((1,2,3))
 >>> id(a)
 >>> a += (3,4,5)
 >>> a
array([4, 6, 8])
 >>> id(a)

So there are two differences: addition is defined differently, and one 
is in-place, and one isn't. Personally, I've always thought that the "in 
place" operators should never work with immutable types: in=place should 
mean in-place,, not "in-place if possible, but not if not possible. and 
yes, this is completely analogous to the issue at hand.

And then we have this (already pointed out in this thread)

 >>> l = [1,2,3]
 >>> l
[1, 2, 3]
 >>> a
array([4, 6, 8])
 >>> l += a
 >>> l
array([ 5,  8, 11])

 >>> b
[4, 6, 8]
 >>> l = [1,2,3]
 >>> l += b
 >>> l
[1, 2, 3, 4, 6, 8]

 >>> l = [1,2,3]
 >>> t = (3,4,5)
 >>> l+=t
 >>> l
[1, 2, 3, 3, 4, 5]

How this is anything but a bug is beyond me! Lists do define the 
in-place operator, and really do it in-place. That being the case, I 
would certainly never expect += to change the type of a list! It doesn't 
with a tuple. Is this a numpy issue or a Python one?

> I don't view it as a problem of 
> function behavior as much as problem with documentation and "mismatch 
> between what a particular user expects and what is actually done."

Quite true. However, what I advocate is that to keep that mismatch as 
rare as possible, the ONLY functions that should have the "sometimes a 
view and sometimes a copy" behavior should be functions that explicitly 
exist to provide that behavior, such as asarray(). All it does is save 
typing and increase the potential for confusion and errors to built it 
in to functions that have other uses.

Travis Oliphant wrote:

> on making methods that return views raise an error when impossible and 
> not changing and/or deprecating functions (i.e. the functions are simple 
> wrappers around the methods).


> on making methods that return views raise an error when impossible and 
> changing the function to make a copy

  +1 : Only if the functions are moved to a separate "backwards 
compatible" name space.


Christopher Barker, Ph.D.
NOAA/OR&R/HAZMAT         (206) 526-6959   voice
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Chris.Barker at noaa.gov

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