[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c (original) (raw)

Travis E. Oliphant oliphant.travis at ieee.org
Tue Mar 28 05:10:10 CEST 2006

Guido van Rossum wrote:

On 3/27/06, Travis Oliphant <oliphant.travis at ieee.org> wrote:

If you have Numeric or numpy installed try this:

#import Numeric as N import numpy as N a = range(10) b = N.arange(10) a.iadd(b) print a Result: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Contrast the returned output with import numpy as N a = range(10) b = N.arange(10) a += b print a Result: [ 0 2 4 6 8 10 12 14 16 18] Having "a+=b" and "a.iadd(b)" do different things seems like an unfortunate bug. It seems to me that the problem is that the INPLACEADD and INPLACEMULTIPLY cases in ceval.c use PyNumberInPlace instead of trying PySequenceInPlace when the object doesn't support the in-place number protocol. I could submit a patch if there is agreement that this is a problem. Well how is the interpreter to know whether += meanse numeric add or sequence add in any particular case?

I can see that '+' (and '*') having two different implementation slots is a design bug. However, it seems prudent that both += and .iadd should have identical behavior regardless. Whatever mechanism is used to resolve the conflict for iadd, the same mechanism should be used for for +=.

I don't think this is a NumPy-only issue. Any object that defines addition that works with lists will have similar problems.

I think this can be fixed easily by first checking the sequence slot for a sq_concat function before calling PyNumber_InPlaceAdd.

All I'm asking for is that a += b have the same behavior as a.iadd(b). That seems like desireable behavior to me.

-Travis

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