On Sun, Jun 3, 2018 at 2:00 PM, Hameer Abbasi <einstein.edi...@gmail.com> wrote:
> The rules for dispatch with ``__array_function__`` match those for > ``__array_ufunc__`` (see > `NEP-13 <http://www.numpy.org/neps/nep-0013-ufunc-overrides.html>`_). > In particular: > > - NumPy will gather implementations of ``__array_function__`` from all > specified inputs and call them in order: subclasses before > superclasses, and otherwise left to right. Note that in some edge cases, > this differs slightly from the > `current behavior <https://bugs.python.org/issue30140>`_ of Python. > - Implementations of ``__array_function__`` indicate that they can > handle the operation by returning any value other than > ``NotImplemented``. > - If all ``__array_function__`` methods return ``NotImplemented``, > NumPy will raise ``TypeError``. > > > I’d like to propose two changes to this: > > - ``np.NotImplementedButCoercible`` be a part of the standard from the > start. > - If all implementations return this, only then should it be > coerced. > - In the future, it might be good to mark something as coercible > to coerce it to ``ndarray`` before passing to another object’s > ``__array_ufunc__``. > - This is necessary if libraries want to keep old behaviour for > some functions, while overriding others. > - Otherwise they have to implement overloads for all functions. > This seems rather like an all-or-nothing choice, which I’d like to > avoid. > - It isn’t too hard to implement in practice. > > I think the issue is real but I would be slightly worried about adding multiple possible things to return - there is a benefit to an answer being either "I cannot do this" or "here's the result". I also am not sure there is an actual problem: In the scheme as proposed, implementations could just coerce themselves to array and call the routine again. (Or, in the scheme I proposed, call the routine again but with `coerce=True`.) > > - Objects that don’t implement ``__array_function__`` should be > treated as having returned ``np.NotImplementedButCoercible``. > - This has the effect of coercing ``list``, etc. > - At a minimum, to maintain compatibility, if all objects don’t > implement ``__array_function__``, the old behaviour should stay. > > I think that in the proposed scheme this is effectively what happens. -- Marten
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