[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c
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 INPLACE_ADD and
>> INPLACE_MULTIPLY cases in ceval.c use PyNumber_InPlace<YYY> instead of
>> trying PySequence_InPlace<YYY> 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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