Bug or Feature?

[Stephan Diehl]

Duncan Booth wrote:

> Stephan Diehl <stephan.diehl at gmx.net> wrote in
> news:bpt5do$nls$04$1 at news.t-online.com:
> 
>> Although, the question was more along the line, if (in the light of
>> the principle of least surprise) this behaviour makes sense.
>> My private opinion would be, that the usefullness of builtin types as
>> real types would be much better, if they were safe under inheritance.
>> 
> There are some tradeoffs here.
> 
> I think that the majority of users would say that the speed of operations
> using 'int' is more important than the ease of inheriting from them. If
> you can make the 'int' operations work this way, but keep them at least as
> fast as they are today, then there may be a case for changing the
> behaviour.

O.K., since the stuff I'm concerned about here is of no relevance for
99.9 % of all python programmers, execution speed is a very valid reason for
this (and I'll consider this behaviour as a feature :-)

> 
> The other problem though is that this would break backwards compatibility.
> For example if any code were to currently add booleans, it would suddenly
> get a boolean result. The (untested) code below, currently counts how many
> items in a list are greater than a specified value, but under your scheme
> it would return a boolean.
> 
>     def HowManyGreaterThan(aList, aValue):
>         return reduce(operator.add, [ x > aValue for x in aList ])

Which, of course, would bring us to the discussion if a bool should be a
subtype of int (and since this was decided long ago, it should not be
discussed, of course)

> 
> I think what would be most useful would be new subclasses of int etc.
> which have the properties you desire, but you can write them yourself
> easily enough.
> 
> What rules, BTW, are you proposing for operations between two separate
> subclasses of int? e.g. given your mod7 class, and a corresponding mod9
> class should this print 3, 6, 24 or throw an exception?
> 
>     x = mod7(3)
>     y = mod9(8)
>     print x*y
> 
> and does this do the same?
> 
>     print y*x
> 

tough question. I'd probably throw an exception.

thanks for your input

Stephan