[Numpy-discussion] Unexpected behaviour by in place addition of array and masked array

[Hanno Klemm]

Hi,

I tried adding a masked array and a usual array in place and got back
the usual array. Is this expected behaviour? I would have expected to
get a masked array back.

The below example illustrates what I mean.


>>> import numpy as N
>>> a = N.zeros((3,3), dtype=float)
>>> b = N.ma.MaskedArray([[1,2,3],[2,3,4],[4,5,6]], mask =
[[0,0,0],[1,0,1], [0,0,0]])
>>> b
array(data =
 [[     1      2      3]
 [999999      3 999999]
 [     4      5      6]],
      mask =
 [[False False False]
 [ True False  True]
 [False False False]],
      fill_value=999999)

>>> a+b
array(data =
 [[  1.00000000e+00   2.00000000e+00   3.00000000e+00]
 [  1.00000000e+20   3.00000000e+00   1.00000000e+20]
 [  4.00000000e+00   5.00000000e+00   6.00000000e+00]],
      mask =
 [[False False False]
 [ True False  True]
 [False False False]],
      fill_value=1e+20)

>>> a+=b
>>> a
array([[ 1.,  2.,  3.],
       [ 0.,  3.,  0.],
       [ 4.,  5.,  6.]])
>>> N.__version__
'1.0.4'
>>> 

Best regards,
Hanno

-- 
Hanno Klemm
klemm at phys.ethz.ch