[Numpy-discussion] Dot as a method

Tim Hochberg tim.hochberg at cox.net
Wed Jul 5 17:14:44 CDT 2006

Bart Vandereycken wrote:
> Hi all,
> reading the thread "Ransom proposals" I was wondering why there isn't a 
> ndarray.dot() method?  There is already a scipy.sparse.dot() so this 
> would fit nicely in the whole idea of polymorphism.
Are you sure about that?

The problem with a dot method (aside from a over proliferation of 
methods in general) is that to be done correctly you want to choose a 
particular implementation of dot based *both* of its arguments. A method 
does a good job dispatching on a single argument, but falls down when 
dispatching on two types. Let's look at this specific case. Imagine that 
in addition ndarray.dot and sparse.dot, we also stick a dot method on 
ma.masked, etc. Now in order to fully exploit polymorphism and get 
maximum efficiency, we want asparsearray.dot(amaskedarray) to correctly 
treat the masked values (ma.dot treats them as zeros) and to not 
instantiate a dense version of the sparsearray. But in order to do that 
all three classes need to know about the other two. That's possible, if 
messy since all three of these are known in advance, but this approach 
becomes untenable if you classes outside core numpy or scipy to 
participate as full citizens.

What does appear fit well here is generic functions / multimethods / 
protocols as discussed in some detail on pydev-3000 a couple of months 
ago. This would allow classes defined outside of core numpy and scipy to 
work correctly and efficiently with dot as long as they register 
appropriate versions of dot. If I wasn't swamped right at the moment I'd 
prototype this up and see how it works in practice.


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