[Numpy-discussion] numexpr does sum.

[Tim Hochberg]

I managed to get basic support for sum and prod into numpexpr. I need to 
tie up some loose ends, for instance only floats are currently 
supported, but these should be easy. To return to the recently posted 
multidimensional distance program, this now works:

    expr = numexpr("sum((a - b)**2, axis=2)", [('a', float), ('b', float)])
    def dist_numexpr(A, B):
        return sqrt(expr(A[:,newaxis], B[newaxis,:]))

It's also quite fast, although there's still room for improvement in the 
reduction code. Notice that it still needs to be in two parts since 
sum/prod needs to surround the rest of the expression. Note also that it 
does support the axis keyword, although currently only nonnegative 
values (or None). I plan to fix that at some point though.

-tim