[Numpy-discussion] replacing the mechanism for dispatching ufuncs

Darren Dale dsdale24@gmail....
Tue Jun 21 13:46:15 CDT 2011

On Tue, Jun 21, 2011 at 2:28 PM, Charles R Harris
<charlesr.harris@gmail.com> wrote:
> On Tue, Jun 21, 2011 at 11:57 AM, Mark Wiebe <mwwiebe@gmail.com> wrote:
>> On Tue, Jun 21, 2011 at 12:36 PM, Charles R Harris
>> <charlesr.harris@gmail.com> wrote:
>>> On Mon, Jun 20, 2011 at 12:32 PM, Mark Wiebe <mwwiebe@gmail.com> wrote:
>>>> NumPy has a mechanism built in to allow subclasses to adjust or override
>>>> aspects of the ufunc behavior. While this goal is important, this mechanism
>>>> only allows for very limited customization, making for instance the masked
>>>> arrays unable to work with the native ufuncs in a full and proper way. I
>>>> would like to deprecate the current mechanism, in particular
>>>> __array_prepare__ and __array_wrap__, and introduce a new method I will
>>>> describe below. If you've ever used these mechanisms, please review this
>>>> design to see if it meets your needs.
>>> The current approach is at a dead end, so something better needs to be
>>> done.
>>>> Any class type which would like to override its behavior in ufuncs would
>>>> define a method called _numpy_ufunc_, and optionally an attribute
>>>> __array_priority__ as can already be done. The class which wins the priority
>>>> battle gets its _numpy_ufunc_ function called as follows:
>>>> return arr._numpy_ufunc_(current_ufunc, *args, **kwargs)
>>>> To support this overloading, the ufunc would get a new support method,
>>>> result_type, and there would be a new global function, broadcast_empty_like.
>>>> The function ufunc.empty_like behaves like the global np.result_type,
>>>> but produces the output type or a tuple of output types specific to the
>>>> ufunc, which may follow a different convention than regular arithmetic type
>>>> promotion. This allows for a class to create an output array of the correct
>>>> type to pass to the ufunc if it needs to be different than the default.
>>>> The function broadcast_empty_like is just like empty_like, but takes a
>>>> list or tuple of arrays which are to be broadcast together for producing the
>>>> output, instead of just one.
>>> How does the ufunc get called so it doesn't get caught in an endless
>>> loop? I like the proposed method if it can also be used for classes that
>>> don't subclass ndarray. Masked array, for instance, should probably not
>>> subclass ndarray.
>> The function being called needs to ensure this, either by extracting a raw
>> ndarray from instances of its class, or adding a 'subok = False' parameter
>> to the kwargs. Supporting objects that aren't ndarray subclasses is one of
>> the purposes for this approach, and neither of my two example cases
>> subclassed ndarray.
> Sounds good. Many of the current uses of __array_wrap__ that I am aware of
> are in the wrappers in the linalg module and don't go through the ufunc
> machinery. How would that be handled?

I contributed the __array_prepare__ method a while back so classes
could raise errors before the array data is modified in place.
Specifically, I was concerned about units support in my quantities
package (http://pypi.python.org/pypi/quantities). But I agree that
this approach is needs to be reconsidered. It would be nice for
subclasses to have an opportunity to intercept and process the values
passed to a ufunc on their way in. For example, it would be nice if
when I did np.cos(1.5 degrees), my subclass could intercept the value
and pass a new one on to the ufunc machinery that is expressed in
radians. I thought PJ Eby's generic functions PEP would be a really
good way to handle ufuncs, but the PEP has stagnated.


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