[Numpy-discussion] A change with minor compatibility questions

[Dag Sverre Seljebotn]

On 10/17/2012 06:56 PM, Dag Sverre Seljebotn wrote:
> On 10/17/2012 05:22 PM, Travis Oliphant wrote:
>> Hey all,
>>
>> https://github.com/numpy/numpy/pull/482
>>
>> is  a pull request that changes the hash function for numpy void
>> scalars.   These are the objects returned from fully indexing a
>> structured array:  array[i] if array is a 1-d structured array.
>>
>> Currently their hash function just hashes the pointer to the underlying
>> data.    This means that void scalars can be used as keys in a
>> dictionary but the behavior is non-intuitive because another void scalar
>> with the same data but pointing to a different region of memory will
>> hash differently.
>>
>> The pull request makes it so that two void scalars with the same data
>> will hash to the same value (using the same algorithm as a tuple hash).
>>      This pull request also only allows read-only scalars to be hashed.
>>
>> There is a small chance this will break someone's code if they relied on
>> this behavior.  I don't believe anyone is currently relying on this
>> behavior -- but I've been proven wrong before.   What do people on this
>> list think?
>
> I support working on fixing this, but if I understand your fix correctly
> this change just breaks things in a different way.
>
> Specifically, in this example:
>
> arr = np.ones(4, dtype=[('a', np.int64)])
> x = arr[0]
> d = { x : 'value' }
> arr[0]['a'] = 4
> print d[x]
>
> Does the last line raise a KeyError? If I understand correctly it does.

Argh. I overlooked both Travis' second commit, and the explicit mention 
of read-only above.

Isn't it possible to produce a read-only array from a writeable one 
though, and so get a read-only scalar whose underlying value can still 
change?

Anyway, sorry about being so quick to post.

Dag Sverre