Ternary plus

[Mark Dickinson]

On Feb 9, 6:47 pm, Martin Drautzburg <Martin.Drautzb... at web.de> wrote:
> BTW I am not really trying to add three objects, I wanted a third object
> which controls the way the addition is done. Sort of like "/" and "//"
> which are two different ways of doing division.

That seems like a reasonable use case for a third parameter to
__add__, though as others have pointed out the only way to pass the
third argument is to call __add__ explicitly.  Here's an extract from
the decimal module:

class Decimal(object):

    ...

    def __add__(self, other, context=None):
        other = _convert_other(other)
        if other is NotImplemented:
            return other

        if context is None:
            context = getcontext()

        <add 'self' and 'other' in context 'context'>

    ...

And here's how it's used in the decimal.Context module:

class Context(object):

    ...

    def add(self, a, b):
        """Return the sum of the two operands.

        >>> ExtendedContext.add(Decimal('12'), Decimal('7.00'))
        Decimal('19.00')
        >>> ExtendedContext.add(Decimal('1E+2'), Decimal('1.01E+4'))
        Decimal('1.02E+4')
        """
        return a.__add__(b, context=self)

--
Mark