How to extarct the mask from a Dask masked array?

Question

In Dask , there is a class called MaskedArray , corresponding to the NumPy class with the same name . The NumPy class has the methods getdata and getmask , which it seems like the Dask class is also supposed to have; however, I can only find a mention of getmask for the Dask class as red text at the getdata documentation page , and when I try to call it in my script, I get

AttributeError: module 'dask.array.ma' has no attribute 'getmask'

So, where is the getmask method for the Dask class? Do I need to obtain the mask in some other way? Or isn't it possible to extract a mask from a Dask masked array at all in the same way as it is possible for a NumPy masked array, and if so, why not? Do I have to convert the Dask masked array to a NumPy masked array before I can extract the mask?

Answer 1

You can use the module method dask.array.ma.getmaskarray . Here's a simple example:

In [2]: import dask.array.ma
   ...: import dask.array
   ...: import numpy as np

In [3]: arr = dask.array.from_array(np.arange(16).reshape(4, 4), chunks=(2, 2))
   ...: mask = dask.array.from_array(np.random.random(size=(4, 4)) > 0.5, chunks=(2, 2))
   ...: masked = dask.array.ma.masked_array(arr, mask)

In [4]: masked
Out[4]: dask.array<masked_array, shape=(4, 4), dtype=int64, chunksize=(2, 2), chunktype=numpy.MaskedArray>

In [5]: da_mask = dask.array.ma.getmaskarray(masked)

In [6]: da_mask
Out[6]: dask.array<getmaskarray, shape=(4, 4), dtype=bool, chunksize=(2, 2), chunktype=numpy.ndarray>

In [7]: da_mask.compute()
Out[7]:
array([[False, False,  True,  True],
       [ True,  True,  True,  True],
       [False,  True, False, False],
       [ True, False,  True, False]])

You can view the list of available module-level methods in the Dask Masked Array API Documentation .