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Tue Mar 28 03:08:00 CEST 2006 · https://mail.python.org/pipermail/python-dev/2006-March/062885.html

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.

-Travis

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Showing content from https://mail.python.org/pipermail/python-dev/2006-March/062885.html below:

[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c