[Numpy-discussion] ndarray special methods should raise NotImplemented instead of TypeError

Charles R Harris charlesr.harris@gmail....
Sun Mar 13 16:45:01 CDT 2011

On Sun, Mar 13, 2011 at 3:24 PM, Hoyt Koepke <hoytak@stat.washington.edu>wrote:

> Apologies for cross posting -- I posted this to the scipy developer
> list a few days ago, since it's really a development question, but
> after getting no response I reread my question and realized it was a
> bit unclear and also was more appropriate for numpy.  So here goes.
> Currently, the numerical special methods of the ndarray class, like
> __add__, __gt__, __mul__, etc., raise a TypeError when they encounter
> a class or type they don't understand.  I do not think this is the
> correct behavior. Instead, they should raise a NotImplemented error.
> Here's the logic behind this.  Classes can also define right operators
> that correspond to each of the above methods -- e.g. __radd__, __le__,
> __rmul__.  However, these are only called under three conditions (see
> http://docs.python.org/reference/datamodel.html#object.__radd__):
> 1. When the left object doesn't implement the left operator.
> 2. When the left object raises a NotImplemented exception.
> 3. When the right object is a subclass of the left object.  In this
> case, the left operator method is never called.
> In my use case, I've been working on implementing a matrix-like class
> of symbolic objects that have to interact seamlessly with numpy
> arrays.  It's a wrapper for CPlex's Concert library, which effectively
> presents variables in an optimization problem as symbolic C++ objects
> that can be used to build the model.  I'm trying to wrap this to work
> with numpy (I'd be happy to share my code when it's done) and the
> current behavior is causing a bit of a headache.
> Consider working with A + X, where A is an ndarray and X is an
> instance of my class that represents my symbolic objects, and let's
> suppose my matrix class implements __radd__ sensibly.  Obviously, 1.
> doesn't apply here.  If I go with 2., a TypeError is raised, which
> blows things apart.  So the only solution I have is 3.
> However, it isn't possible for me to use the underlying array
> machinery provided by ndarray, as the python class simply wraps other
> C++ containers.  Thus, if I subclass ndarray, the underlying array is
> meaningless -- while I do implement slicing and various other
> array-like methods, the only reason I'm subclassing ndarray is (3).
> The big problem with this is that many of the attributes and methods
> of ndarray don't make sense for my class, so I have to cover them up
> with dummy methods that simply raise exceptions.  Doable, but far from
> ideal.
> For now, I'm doing (3) to get it to work, but it would be really nice
> if ndarray would raise a NotImplemented error instead of a TypeError.
> Is there any reason / use case justifying the current behavior?  Would
> it be possible/easy to change it?
You can also define __array_priority__  in your class. If your class isn't a
subclass, then numpy will always call your class methods whether or not your
class is on the right or left. Sounds like you don't want to sublass ndarray
here so that should work.

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