[Numpy-discussion] Help on subclassing numpy.ma: __array_wrap__

[Kevin Dunn]

Hi everyone,

I'm subclassing Numpy's MaskedArray to create a data class that handles
missing data, but adds some extra info I need to carrry around. However I've
been having problems keeping this extra info attached to the subclass
instances after performing operations on them.

The bare-bones script that I've copied here shows the basic issue:
http://pastebin.com/f69b979b8  There are 2 classes: one where I am able to
subclass numpy (with help from the great description at
http://www.scipy.org/Subclasses), and the other where I subclass numpy.ma,
using the same ideas again.

When stepping through the code in a debugger, lines 76 to 96, I can see that
the numpy subclass, called DT, calls DT.__array_wrap__() after it completes
unary and binary operations. But the numpy.ma subclass, called DTMA, does
not seem to call DTMA.__array_wrap__(), especially line 111.

Just to test this idea, I overrode the __mul__ function in my DTMA subclass
to call DTMA.__array_wrap__() and it returns my extra attributes, in the
same way that Numpy did.

My questions are:

(a) Is MA intended to be subclassed?

(b) If so, perhaps I'm missing something to make this work.  Any pointers
will be appreciated.

So far it seems the only way for me to sub-class numpy.ma is to override all
numpy.ma functions of interest for my class and add a DTMA.__array_wrap()
call to the end of them.  Hopefully there is an easier way.

Related to this question, was there are particular outcome from this
archived discussion (I only joined the list recently):
http://article.gmane.org/gmane.comp.python.numeric.general/24315  because
that dictionary object would be exactly what I'm after here.

Thanks,

Kevin
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