__rpow__ issues

[Chris Smith]

I have a routine for converting an expression into an AccumBounds which is something like

def as_bounds(e):

    if e.is_Symbol:

        return appropriate AccumBounds

    elif e.is_number or not e.args:

        return e

    return e.func(*[bounds(i) for i in e.args

When I call it like this, I don't get an unevaluated power

>>> as_bounds(.5**hint('ip'))

0.5**AccumBounds(1, oo)

But I have an __rpow__ defined for AccumBounds and it works

>>> 0.5**AccumBounds(1, oo)

AccumBounds(0, 0.5)

So what do I need to understand to explain why the result of as_bounds is not evaluated to an AccumBounds instance?

/c

[Aaron Meurer]

I think the problem is that Pow doesn't look at __rpow__ (none of the
sympy classes call the operator methods like this).

We could make Accumbounds use the postprocessors to dispatch on Pow. I
tried doing this at https://github.com/sympy/sympy/pull/13161 for Mul
and Add, but the processors were not well designed at the time. Now
they are designed better. Although Pow is a two-argument function, so
we could also look at making it use multipledispatch. That would be a
more complicated change, however. Right now, the base can dispatch in
Pow by defining _eval_power, but I think the only way the exponent can
dispatch is by using the postprocessors.

Aaron Meurer

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[Chris Smith]

Thanks, Aaron.