[Numpy-discussion] Changing MaskedArray.squeeze() to never return masked

[Allan Haldane]

On 07/18/2017 09:52 AM, Benjamin Root wrote:
> This sort of change seems very similar to the np.diag() change a few years
> ago. Are there lessons we could learn from then that we could apply to here?
> 
> Why would the returned view not be a masked array?
> 
> Ben Root

I am in favor of the proposed change below.

I'd like to merge it, but before that I want to make sure I understand
your comment.

Are you referring to the proposed change to make diag return a view
instead of a copy? Note that this has not actually happened yet:
https://github.com/numpy/numpy/issues/7661

Also, I think this case is different because it does not change core
numpy, rather this is to make the MaskedArray module act more
consistently with core numpy. Because of that I think it is much less
problematic than the diag changes.

Cheers,
Allan


> On Tue, Jul 18, 2017 at 9:37 AM, Eric Wieser <wieser.eric+numpy at gmail.com>
> wrote:
> 
>> When using ndarray.squeeze, a view is returned, which means you can do
>> the follow (somewhat-contrived) operation:
>>
>>>>> def fill_contrived(a):
>>         a.squeeze()[...] = 2
>>         return a
>>>>> fill_contrived(np.array([1]))
>> array(2)
>>
>> However, when tried with a masked array, this can fail, breaking liskov
>> subsitution:
>>
>>>>> fill_contrived(np.ma.array([1], mask=[True]))
>> MaskError: Cannot alter the masked element.
>>
>> This fails because squeeze breaks the contract of returning a view,
>> instead deciding sometimes to return masked.
>>
>> There is a patch that fixes this in gh-9432
>> <https://github.com/numpy/numpy/pull/9432> - however, by necessity it
>> breaks any existing code that uses m_arr.squeeze() is np.ma.masked.
>>
>> Is this too breaking a change?
>>
>> Eric
>> ​
>>
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> 
> 
> 
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