On Sat, Oct 17, 2009 at 6:49 AM, Darren Dale <dsdal...@gmail.com> wrote:
> numpy's functions, especially ufuncs, have had some ability to support > subclasses through the ndarray.__array_wrap__ method, which provides > masked arrays or quantities (for example) with an opportunity to set > the class and metadata of the output array at the end of an operation. > An example is > > q1 = Quantity(1, 'meter') > q2 = Quantity(2, 'meters') > numpy.add(q1, q2) # yields Quantity(3, 'meters') > > At SciPy2009 we committed a change to the numpy trunk that provides a > chance to determine the class and some metadata of the output *before* > the ufunc performs its calculation, but after output array has been > established (and its data is still uninitialized). Consider: > > q1 = Quantity(1, 'meter') > q2 = Quantity(2, 'J') > numpy.add(q1, q2, q1) > # or equivalently: > # q1 += q2 > > With only __array_wrap__, the attempt to propagate the units happens > after q1's data was updated in place, too late to raise an error, the > data is now corrupted. __array_prepare__ solves that problem, an > exception can be raised in time. > > Now I'd like to suggest one more improvement to numpy to make its > functions more generic. Consider one more example: > > q1 = Quantity(1, 'meter') > q2 = Quantity(2, 'feet') > numpy.add(q1, q2) > > In this case, I'd like an opportunity to operate on the input arrays > on the way in to the ufunc, to rescale the second input to meters. I > think it would be a hack to try to stuff this capability into > __array_prepare__. One form of this particular example is already > supported in quantities, "q1 + q2", by overriding the __add__ method > to rescale the second input, but there are ufuncs that do not have an > associated special method. So I'd like to look into adding another > check for a special method, perhaps called __input_prepare__. My time > is really tight for the next month, so I'd rather not start if there > are strong objections, but otherwise, I'd like to try to try to get it > in in time for numpy-1.4. (Has a timeline been established?) > > I think it will be not too difficult to document this overall scheme: > > When calling numpy functions: > > 1) __input_prepare__ provides an opportunity to operate on the inputs > to yield versions that are compatible with the operation (they should > obviously not be modified in place) > > 2) the output array is established > > 3) __array_prepare__ is used to determine the class of the output > array, as well as any metadata that needs to be established before the > operation proceeds > > 4) the ufunc performs its operations > > 5) __array_wrap__ provides an opportunity to update the output array > based on the results of the computation > > Comments, criticisms? If PEP 3124^ were already a part of the standard > library, that could serve as the basis for generalizing numpy's > functions. But I think the PEP will not be approved in its current > form, and it is unclear when and if the author will revisit the > proposal. The scheme I'm imagining might be sufficient for our > purposes. > > This sounds interesting to me, as it would push the use of array wrap down into a common function and make it easier to use. I wonder what the impact would be on the current subclasses of ndarray? On a side note, I wonder if you could look into adding your reduce loop optimizations into the generic loops? It would be interesting to see if that speeded up some common operations. In any case, it can't hurt. Chuck
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