[Python-Dev] Mini-Pep: Simplifying the Integral ABC

[Guido van Rossum]

On Fri, Jun 6, 2008 at 11:40 AM, Alex Martelli <aleaxit at gmail.com> wrote:
> On Fri, Jun 6, 2008 at 11:01 AM, Guido van Rossum <guido at python.org> wrote:
>> On Thu, Jun 5, 2008 at 8:45 PM, Raymond Hettinger <python at rcn.com> wrote:
>>> Does anyone actually need an int lookalike with binary methods but
>>> cannot just inherit from int?
>>
>> Does anyone actually need an int lookalike with operations like +, -
>> etc. but cannot just inherit from int? If the answer is yes, is there
>> a compelling reason why they wouldn't want to support binary methods
>> as well?
>
> Yes, there's a use case for implementing long integers as arrays of
> decimal digits -- addition is roughly as efficient as for binary
> integers (x86 chips still have instructions to help with that), and
> emitting as decimal digits is MUCH more efficient of course -- so if
> I/O in decimal form is the most common operation, with a little
> arithmetic (particularly sums), you could gain performance;

I tried this with ABC in '83. We didn't see any of the hoped-for
benefits though. That's why Python has binary long integers. :-)

> binary
> operations, however, would be as inefficient as decimal form
> conversion is for ordinary binary ints, and not needed for the typical
> applications that would use these "decimal coded integers"
> (accounting), so why not save the implementer of such an extension
> from having to write that unneeded and slow extra code?

You could just raise an exception. This is common in Java when an
Interface requires you implement a method you can't. Or use virtual
inheritance from the Integral class and leave them unimplemented. See
if anyone cares. :-)

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)