[Numpy-discussion] {zeros, empty, ...}_like functions behavior with subclasses

Wes McKinney wesmckinn@gmail....
Mon Feb 28 21:52:41 CST 2011


On Mon, Feb 28, 2011 at 7:24 PM, Pierre GM <pgmdevlist@gmail.com> wrote:
>
> On Mar 1, 2011, at 1:05 AM, Bruce Southey wrote:
>
>> On Mon, Feb 28, 2011 at 4:52 PM, Wes McKinney <wesmckinn@gmail.com> wrote:
>>> I'm having some trouble with the zeros_like function via np.fix:
>>>
>>> def zeros_like(a):
>>>    if isinstance(a, ndarray):
>>>        res = ndarray.__new__(type(a), a.shape, a.dtype, order=a.flags.fnc)
>>>        res.fill(0)
>>>        return res
>>>    try:
>>>        wrap = a.__array_wrap__
>>>    except AttributeError:
>>>        wrap = None
>>>    a = asarray(a)
>>>    res = zeros(a.shape, a.dtype)
>>>    if wrap:
>>>        res = wrap(res)
>>>    return res
>>>
>>> As you can see this is going to discard any metadata stored in a
>>> subtype. I'm not sure whether this is a bug or a feature but wanted to
>>> bring it up.
>>>
>>> Thanks,
>>> Wes
>>> _______________________________________________
>>> NumPy-Discussion mailing list
>>> NumPy-Discussion@scipy.org
>>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>>>
>>
>> I guess this is ticket 929.
>> http://projects.scipy.org/numpy/ticket/929
>>
>> I was looking at it today but was not sure what is really desired
>> here. I considered that this just meant shape and dtype but not sure
>> about masked or record arrays behavior. So:
>> What is the value of having the metadata?
>> What is the meaning of 'like' here?
>
> Well, that depends on what you wanna do, of course. To handle metadata, I use some kind of dictionary updated in the __array_finalize__. Check numpy.ma.MaskedArray and its subclasses (like scikits.timeseries.TimeSeries) for the details.
> Now that you could store some extra data in the dtype (if I remmbr and understand correctly), it might be worth considering a proper way to deal with that.
>
>
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>

The ticket is exactly related to the problem at hand-- having
__array_finalize__ defined won't help you as it never gets called.


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