when 'myArray * 'myObject' is not equal to 'myObject' * 'myArray'

Duim bauketilma at gmail.com
Thu Aug 19 10:56:02 CEST 2010


On Aug 18, 10:03 pm, Nobody <nob... at nowhere.com> wrote:
> On Wed, 18 Aug 2010 05:56:27 -0700, Duim wrote:
> > Although I'm sure somewhere this issue is discussed in this (great)
> > group, I didn't know the proper search words for it (although I
> > tried).
>
> > I'm using python (2.6) scientifically mostly, and created a simple
> > class to store time series (my 'Signal' class).
> > I need this class to have a possibility to get multiplied by an array,
> > but pre and post multiplication have different mathematical outcomes
> > ( basically A* B != B*A ) .
>
> > Post multiplication by an array works fine defining __mul__ in the
> > Signal class, but pre multiplication does not. It keeps trying to
> > multiply all elements separately instead to send this array to my
> > __rmul__ function.
>
> > How can I fix this without the need for a separate
> > 'multiplysignal(A,B)' function?
>
> Make Signal a subclass of numpy.ndarray. If one operand is a subclass of
> the other, its __rmul__ will be preferred to the parent's __mul__.
>
> In the absence of a subclass-superclass relationship, the LHS's __mul__ is
> preferred to the RHS's __rmul__, so the RHS's __rmul__ is only called if
> the LHS lacks a __mul__ method or if the method refuses its argument
> (returns NotImplemented).
>
> Likewise for other "reflected" methods.

Great, many thanks. It seems to work well.
For others looking into the same issue: http://www.scipy.org/Subclasses




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