[Numpy-discussion] array += masked_array gives a normal array, not masked array

[Tony Yu]

Ok, so you guys shot down my last attempt at finding a bug :). Here's  
another attempt.

      array + masked_array

outputs a masked array

      array += masked_array

outputs an array.

I'm actually not sure if this is a bug (works the same for both the  
old and new masked arrays), but I thought this was unexpected.

*However*, if the right side were an old masked array, the masked  
values would not be added to the output array. With the new masked  
arrays, the data from the masked array is added regardless of whether  
the value is masked (example, below).

-Tony

Example:
=======

In [1]: import numpy

In [2]: normal = numpy.array([1, 1])

In [3]: masked = numpy.ma.array([1, 1], mask=[True, False])

In [4]: new_masked = normal + masked

In [5]: new_masked

Out[5]:
masked_array(data = [-- 2],
       mask = [ True False],
       fill_value=999999)


In [6]: normal += masked

In [7]: normal

Out[7]: array([2, 2])

# If used old masked arrays in the above, the final output would be:  
array([1, 2])