[SciPy-dev] Maskedarray implementations

[Pierre GM]

All,

As you might be aware, there are currently two concurrent implementations of 
masked arrays in numpy:
* numpy.ma is the official implementation, but it is unclear whether it is 
still actively maintained.
* maskedarray is the alternative I've been developing initially for my own 
purpose from numpy.ma. It is available in the scipy svn sandbox, but is 
already fully functional

The main difference between numpy.ma and maskedarray is that the objects 
created by numpy.ma are NOT ndarrays, while maskedarray.MaskedArray is a full 
subclass of ndarrays. For example:

>>>import numpy, maskedarray
>>>x = numpy.ma.array([1,2], mask=[0,1])
>>>isinstance(x, numpy.ndarray)
False
>>>numpy.asanyarray(x)
array([1,2])
Note that we just lost the mask...

>>>x = maskedarray.array([1,2], mask=[0,1])
>>>isinstance(x, numpy.ndarray)
True
>>>numpy.asanyarray(x)
masked_array(data = [1 --],
      mask = [False  True],
      fill_value=999999)
Note that the mask is conserved.

Having the masked array be a subclass of ndarray makes masked arrays easier to 
mix with other ndarray types and to subclass. An example of application is 
the TimeSeries package, where the main TimeSeries class is a subclass of 
maskedarray.MaskedArray.

* Does anyone see any *disadvantages* to this aspect of maskedarray relative 
to numpy.ma?

* What would be the requisites to move maskedarray out of the sandbox ? We 
hope to be able in the short term to either replace or at least merge the two 
implementations, once a couple of issues are addressed (but we can talk about 
that later...)

Thanks a lot in advance for your feedback
Pierre