numpy.ma.MaskedArray.__setitem__
method
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MaskedArray.__setitem__(indx, value)[source]
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x.__setitem__(i, y) <==> x[i]=y Set item described by index. If value is masked, masks those locations. 
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Licensed under the 3-clause BSD License.
    https://docs.scipy.org/doc/numpy-1.16.1/reference/generated/numpy.ma.MaskedArray.__setitem__.html