BUG: `np.ma.allequal` fails when comparing masked array with itself

[MatejUrbanQC]

Describe the issue:

np.ma.allequal(a, a, fill_value=False) should return True if a is a masked array with the mask being all False

It works if the mask is nomask but not if it is [False, False, ...]

The bug only occurs if the masks of the two arrays are object equal (see example)

Reproduce the code example:

import numpy as np
a = np.ma.masked_array([1,2,3], mask=[0,0,0])
b = np.ma.masked_array([1,2,3], mask=[0,0,0])
assert np.ma.allequal(a, b, fill_value=False) # this works
assert np.ma.allequal(a, a, fill_value=False) # this fails

Error message:

No response

Python and NumPy Versions:

numpy: 2.0.0
python: 3.12.4 | packaged by conda-forge | (main, Jun 17 2024, 10:13:44) [Clang 16.0.6 ]

Runtime Environment:

No response

Context for the issue:

No response

[ianharris]

I had a look at this. The np.ma.allequal function contains three conditional blocks. An if, and elif, and an else. In the failing case above the else block is being executed, which simply returns False. The elif block is not executed as it's conditional is the boolean fill_value. And the if statement is not executed because in this case, the condition can never be True - which I'm not sure is the intention.

The issue seems to arise from the np.ma.mask_or function which is called to make a logical OR mask from the masks of the two masked arrays input into allequal. In the specific case where the two input masked arrays are the same, mask_or will return the mask of the first input array. In other cases, mask_or will try to "shrink" the mask. Shrinking the masks in the examples above would give the value np.ma.nomask.

Returning to the if statement in allequal the condition is that the logical OR mask has the value nomask. I believe there are three possible solutions:

1. Modify mask_or so that it tries to shrink the mask in this case. This could potentially break other functionality though.
2. Call _shrink_mask on the mask returned from the call to mask_or within the allequal function.
3. Add a conditional check to the start of allequal to check if the two input masked arrays reference the same object.

I'm interested in contributing so happy to implement a fix. But it would be good to get an opinion from a more seasoned contributor about which strategy to pursue.

[mattip]

#27360 closed this. Please reopen or open a new issue if there is still more to do.