Dask.array deepcopy does not preserve masking since 0.18.2 #3848
Description
If 'a' is a dask array wrapped around a numpy masked array (with "from _array(..., asarray=False)",
then the result of "copy.deepcopy(a).compute()" is now (wrongly) not a masked array,
Though the original "a.compute()", and even "copy.copy(a)" are correct.
Example code to show :
import dask.array as da
import numpy.ma as ma
import copy
t = ma.masked_array([1, 2], mask=[0, 1])
a = da.from_array(t, chunks=t.shape, asarray=False)
print(copy.deepcopy(a).compute())
For example, in dask 0.18.1
>>> import dask
>>> import dask.array as da
>>> import numpy.ma as ma
>>> import copy
>>> t = ma.masked_array([1, 2], mask=[0, 1])
>>> a = da.from_array(t, chunks=t.shape, asarray=False)
>>> print(dask.__version__)
0.18.1
>>> print(copy.copy(a).compute())
[1 --]
>>> print(copy.deepcopy(a).compute())
[1 --]
>>>
But in 0.18.2 ...
>>> import dask
>>> import dask.array as da
>>> import numpy.ma as ma
>>> import copy
>>>
>>> t = ma.masked_array([1, 2], mask=[0, 1])
>>> a = da.from_array(t, chunks=t.shape, asarray=False)
>>>
>>> print(dask.__version__)
0.18.2
>>> print(copy.copy(a).compute())
[1 --]
>>> print(copy.deepcopy(a).compute())
[1 2]
>>>