[Python-Dev] Advice on numbers.py implementation of binary mixins.

[Jim Jewett]

Raymond Hettinger wrote:

> PEP-3141 outlines an approach to writing binary
> operators to allow the right operand to override
> the operation if the left operand inherits the
> operation from the ABC.

> Here is my first approximation at how to write
> them for the Integral mixins:

> class Integral(Rational):

>    def __and__(self, other):
>        if isinstance(other, (type(self), int, long)):  # XXX
>            return int(self) & int(other)

I think for this mixin, it doesn't matter whether other is an Integral
instance; it matter whether it is has a more specific solution.

So instead of checking whether isinstance, check whether its __rand__
method is Integral.__rand__.

I think you also may want to guard against incomplete right-hand
operations, by doing something like replacing the simple

>     return NotImplemented

with

    try:
        val = other.__rand__(self)
        if val is not NotImplemented:
            return val
    except (TypeError, AttributeError):
        pass
    # Use the generic fallback after all
    return int(self) & int(other)