[Numpy-discussion] ndarray.fill and ma.array.filled

Eric Firing efiring at hawaii.edu
Wed Mar 22 15:38:01 CST 2006

Sasha wrote:
> In an ideal world, any function that accepts ndarray would accept
> ma.array and vice versa.  Moreover, if the ma.array has no masked
> elements and the same data as ndarray, the result should be the same. 

This would be *very* nice.

> Obviously current implementation falls short of this goal, but there
> is one feature that seems to make this goal unachievable.
> This feature is the "filled" method of ma.array.  Pydoc for this
> method reports the following:
>  |  filled(self, fill_value=None)
>  |      A numeric array with masked values filled. If fill_value is None,
>  |                 use self.fill_value().
>  |
>  |                 If mask is nomask, copy data only if not contiguous.
>  |                 Result is always a contiguous, numeric array.
>  |      # Is contiguous really necessary now?
> That is not the best possible description ("filled" is "filled"), but
> the essence is that the result of a.filled(value) is a contiguous
> ndarray obtained from the masked array by copying non-masked elements
> and using value for masked values.
> I would like to propose to add a "filled" method to ndarray.  I see
> several possibilities and would like  to hear your opinion:
> 1. Make filled simply return self.
> 2. Make filled return a contiguous copy.
> 3. Make filled replace nans with the fill_value if array is of
> floating point type.

It seems to me that any function or method that returns an array from an 
array should be perfectly consistent and explicit about whether it makes 
a copy or not.  Sometimes the filled method *needs* to return a copy; 
therefore it should *always* return a copy, regardless of the presence 
or state of masking. Hence I think the filled method of ma needs to be 
changed in this way also.

The question for your suggestion 3 is, should a nan always be the 
equivalent of a masked value?  One loses a little flexibility, but it 
has an appealing simplicity to it.  I could be persuaded otherwise, but 
right now I would vote for it.


> Unfortunately, adding "filled" will result is a rather confusing
> situation where "fill" and "filled" both exist and have very different
> meanings.
> I would like to note that "fill" is a somewhat odd ndarray method.
> AFAICT, it is the only non-special method that mutates the array.  It
> appears to be just a performance trick: the same result can be achived
> with "a[...] = ".
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