set_ndv does not update nodatas in array

[rhugonnet]

During set_ndv, the array is not updated with previous nodata.

Steps to reproduce the behavior:

r = gu.Raster(landsat_b4_path)
r.data.ravel()[0:100]=255
r.set_ndv(255)
np.max(r.data)
Out[22]: 254
np.max(r.data.data)
Out[23]: 255
r.set_ndv(254, update_array=True)
np.max(r.data.data)
Out[25]: 255

All the 255 no data values should have been updated to 254 by the set_ndv(..., update_array=True), which is also the default parameter.

[adehecq]

I don't think it's a bug. Maybe we should clarify in the docstring, but looking at the code, it seems like the behavior of set_ndv is meant to be:

• update/set self.nodata
• if update_array=True, unmask old masked values, mask values currently set to nodata.

This is exactly what is done in your example, i.e. the 255 values are masked, then unmasked.
We just have to agree on what is the typical case in which one would use set_ndv. I don't think the example that you show is something that one would typically want to do, why set a nodata value and then change it later?

It also raises the broader question. With masked_arrays, the masked values are actually not being modified. If one runs self.set_mask, self.data.mask is modified but not self.data.data. This can be an issue with certain rasterio's functions actually that do not use the mask.

[adehecq]

Ok, after discussions, I changed my mind.
We need to update the behavior so that it is possible to update the nodata value while keeping the same mask (useful if a certain nodata value is not appropriate, e.g. because it is used).
Also, the current functionality of update_array=True does not make sense to me. Masked pixels can contain any value, so when updating the array, we should make sure that currently masked values are set to the previous nodata value.
In summary, we need to :

• update update_array=True so that the previously masked pixels are filled with the previous nodata value (e.g. to reverse an accident in a set_ndv call)
• add an option preserve_mask set to True by default, which keeps the previous mask. I'm unsure what to do if values are set to the new nodata. Maybe raise a warning?