[Numpy-discussion] Implementing hashing protocol for dtypes

David Cournapeau cournape@gmail....
Wed Mar 11 22:49:19 CDT 2009


On Thu, Mar 12, 2009 at 5:36 AM, Robert Kern <robert.kern@gmail.com> wrote:
> On Wed, Mar 11, 2009 at 15:06, David Cournapeau <cournape@gmail.com> wrote:
>> Hi,
>>
>> I was looking at #936, to implement correctly the hashing protocol for
>> dtypes. Am I right to believe that tp_hash should recursively descend
>> fields for compound dtypes, and the hash value should depend on the
>> size/ndim/typenum/byteorder for each "atomic" dtype + fields name (and
>> titles) ? Contrary to comparison, we can't reuse the python C api,
>> since PyObject_Hash cannot be applied to the fields dict, right ?
>
> Usually, one constructs a hashable analogue; e.g. taking the .descr
> and converting all of the lists to tuples. Then use PyObject_Hash on
> that.

Is the .descr of two dtypes guaranteed to be equal whenever the dtypes
are equal ? It is not obvious to me that PyArray_EquivTypes is
equivalent to comparing the descr ?

David


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