[Numpy-discussion] MA bug or feature?

Michael Sorich michael.sorich at gmail.com
Wed Jun 21 21:01:59 CDT 2006

I was setting the fill_value as 'NA' when constructing the array so
the masked values would be printed as 'NA'. It is not a big deal to
avoid doing this.

Nevertheless, the differences between a masked array with a boolean
mask and a mask of booleans have caused me trouble before. Especially
when there are hidden in-place conversions of a mask which is a array
of False to a mask which is False. e.g.

import numpy
print numpy.version.version

ma1 = numpy.ma.array(((1.,2,3),(4,5,6)), mask=((0,0,0),(0,0,0)))
print ma1.mask
a1 = numpy.asarray(ma1)
print ma1.mask

[[False False False]
 [False False False]]

On 6/21/06, Pierre GM <pgmdevlist at mailcan.com> wrote:
> On Wednesday 21 June 2006 04:46, Michael Sorich wrote:
> > When transposing a masked array of dtype '<f8' I noticed that an
> > ndarray of dtype '|O4' was returned.
> OK, I see where the problem is:
> When your fill_value has a type that cannot be converted to the type of your
> data, the `filled` method (used internally in many functions, such as
> `transpose`) raises a TypeError, which is caught and your array is converted
> to 'O'.
> That's what happen here: your fill_value is a string, your data are integer,
> the types don't match, hence the conversion. So, no, I don't think that's a
> bug.
> Why filling when you don't have any masked values, then ? Well, there's a
> subtle difference between a boolean mask and a mask of booleans.
> When the mask is boolean (mask=nomask=False), there's no masked value, and
> `filled` returns the data.
> Now, when your mask is an array of boolean (your first case), MA doesn't check
> whether mask.any()==False to determine whether there are some missing data or
> not, it just processes the whole array of boolean.
> I agree that's a bit confusing here, and there might be some room for
> improvement (for example, changing the current
> `if m is nomask` to `if m is nomask or m.any()==False`, or better, forcing
> mask to nomask if mask.any()==False). But I don;t think that qualifies as
> bug.
> In short:
> when you have an array of numbers, don't try to fill it with characters.

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