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np.ma.masked_where(copy=False) fails to modify array in place when there were no masked elements in the array initially #18946

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jacekt opened this issue May 8, 2021 · 1 comment · Fixed by #18967
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np.ma.masked_where(copy=False) fails to modify array in place when there were no masked elements in the array initially #18946

jacekt opened this issue May 8, 2021 · 1 comment · Fixed by #18967
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00 - Bug component: numpy.ma masked arrays
Milestone
1.21.0 release

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@jacekt
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jacekt commented May 8, 2021 •
edited by rgommers
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ma.masked_where documentation says that:

copy : bool
    If True (default) make a copy of `a` in the result.  If False modify
    `a` in place and return a view.

But it fails to modify the array's mask when the original mask is empty:

Reproducing code example:

>>> import numpy as np
>>> import numpy.ma as ma

>>> a = ma.array([1, 2, 3, 4])
>>> a
masked_array(data=[1, 2, 3, 4],
             mask=False,
       fill_value=999999)

>>> ma.masked_where(a==3, a, copy=False)
masked_array(data=[1, 2, --, 4],
             mask=[False, False,  True, False],
       fill_value=999999)

>>> a
masked_array(data=[1, 2, 3, 4],
             mask=False,
       fill_value=999999)

It works properly when there are some masked elements in the array already:

>>> a = ma.array([1, 2, 3, 4], mask=[1, 0, 0, 0])
>>> a
masked_array(data=[--, 2, 3, 4],
             mask=[ True, False, False, False],
       fill_value=999999)

>>> ma.masked_where(a==3, a, copy=False)
masked_array(data=[--, 2, --, 4],
             mask=[ True, False,  True, False],
       fill_value=999999)

>>> a
masked_array(data=[--, 2, --, 4],
             mask=[ True, False,  True, False],
       fill_value=999999)

Error message:

None.

NumPy/Python version information:

1.20.2 3.8.6 (tags/v3.8.6:db45529, Sep 23 2020, 15:52:53) [MSC v.1927 64 bit (AMD64)]
numpy-1.20.2-cp38-cp38-win_amd64.whl
@jacekt jacekt changed the title np.ma.masked_where(copy=False) fails to modify array in place when there are no masked elements in the array np.ma.masked_where(copy=False) fails to modify array in place when there were no masked elements in the array initially May 9, 2021
@constanzafierro constanzafierro mentioned this issue May 9, 2021
Merged
@rgommers rgommers added this to the 1.21.0 release milestone May 10, 2021
rgommers pushed a commit that referenced this issue May 10, 2021
@constanzafierro
BUG: fix np.ma.masked_where(copy=False) when input has no mask (#18967)
b6eb3d8 
Fixes gh-18946
@rgommers
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Thanks for the report @jacekt. Fixed now.

pbrod pushed a commit to pbrod/numpy that referenced this issue May 11, 2021
@constanzafierro @pbrod
BUG: fix np.ma.masked_where(copy=False) when input has no mask (numpy…
2a42ccb 
…#18967)

Fixes numpygh-18946
@QuLogic QuLogic mentioned this issue Jun 24, 2021
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BUG: fix np.ma.masked_where(copy=False) when input has no mask constanzafierro/numpy
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