[Numpy-discussion] Bringing order to higher dimensional operations

Sebastian Berg sebastian at sipsolutions.net
Thu Jul 18 09:23:39 EDT 2013


On Thu, 2013-07-18 at 13:52 +0100, Nathaniel Smith wrote:
> Hi all,
> 
<snip>
> 
> So:
> 
> QUESTION 1: does that sound right: that in a perfect world, the
> current gufunc convention would be the only one, and that's what we
> should work towards, at least in the cases where that's possible?
> 

Sounds right to me, ufunc/gufunc broadcasting assumes the "inner"
dimensions are the right-most. Since we are normally in C-order arrays,
this also seems the sensible way if you consider the memory layout.

> QUESTION 2: Assuming that's right, it would be *really nice* if we
> could at least get np.dot onto our new convention, for consistency
> with the rest of np.linalg, and to allow it to be overridden. I'm sort
> of terrified to touch np.dot's API, but the only cases where it would
> act differently is when *both* arguments have *3 or more dimensions*,
> and I guess there are very very few users who fall into that category.
> So maybe we could start raising some big FutureWarnings for this case
> in the next release, and eventually switch?
> 

It is noble to try to get do to use the gufunc convention, but if you
look at the new gufunc linalg functions, they already have to have some
weird tricks in the case of np.linalg.solve.
It is so difficult because of the fact that dot is basically a
combination of many functions:
  o vector * vector -> vector
  o vector * matrix -> matrix (add dimensions to vector on right)
  o matrix * vector -> matrix (add dimensions to vector on left)
  o matrix * matrix -> matrix
plus scalar cases.

I somewhat believe we should not touch dot, or deprecate anything but
the most basic dot functionality. Then we can point to matrix_multiply,
inner1d, etc. which are gufuncs (even if they are not exposed at this
time). The whole dance that is already done for np.linalg.solve right
now is not pretty there, and it will be worse for dot. Because dot is
basically overloaded, marrying it with the broadcasting machinery in a
general way is impossible.

> (I'm even more terrified of trying to mess with np.sum or
> np.add.reduce, so I'll leave that alone for now -- maybe we're just
> stuck with them. And at least they do already go through the ufunc
> machinery.)

I did not understand where the inconsistency/problem for the reductions
is.

- Sebastian

> 
> -n
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