On 2006-09-27, Steven D'Aprano <[EMAIL PROTECTED]> wrote:
> On Wed, 27 Sep 2006 13:37:40 +0000, Paul McGuire wrote:
>
>> Here's a little more refined version of my previous post:
>>
>> class StubbornInt(int):
>> def invariantOp(self,other):
>> return self
>> __add__ = invariantOp
>> __sub__ = invariantOp
>
> [snip rest of code]
>
>> import sys
>> Top = StubbornInt(sys.maxint)
>
> Top shouldn't need to depend on a specific integer value, but then it is
> easy enough to subclass StubbornInt.
>
> However, there is a more serious problem with your definition of Top:
>
>>>> Top > sys.maxint+1
> False
>
> But Top is supposed to bigger than everything, right?
>
> Okay, let's fix it:
>
> class FixedTop(StubbornInt):
> def __init__(self):
> super(FixedTop, self).__init__(sys.maxint)
> self.name = 'Top'
> def _bigger(self, other):
> # Top is bigger than everything
> return True
> def _smaller(self, other):
> # Top is smaller than nothing
> return False
> __lt__ = __le__ = _smaller
> __gt__ = __ge__ = _bigger
>
>
> Let's test it out:
>
>>>> Top = FixedTop()
>>>> Top > 45
> True
>>>> Top > sys.maxint
> True
>>>> Top > 2L**512
> True
>>>> Top > None
> True
>>>> Top > "hello"
> True
>
>
> So far so good.
>
> However, the downside of rolling your own Top is the risk that one day
> you'll have something like this:
>
> class Funny:
> def __init__(self, name):
> self.name = name
> def __str__(self):
> return self.name
> def __gt__(self, other):
> return len(str(self)) > len(str(other))
>
> and then you'll have problems:
>
>>>> bottom = Funny("Bottom")
>>>> Top > bottom
> True
>>>> bottom > Top
> True
>
> As far as I can see, the only way to avoid edge cases like this is for Top
> (and Bottom) to be built-ins that are special-cased by Python.
An other problem is that if you roll your own, they wont be accepted in
a lot of buitins where they would make sense. Take itertools.repeat as
it is now and I understand correctly there are two possibilities.
1) Your extreme values are not a subclass of int, then you won't be able
to use them as a times arguments.
2) Your extreme values are a subclass of int, then they will somehow
have an int value and this int value will be used, so that the repeat
won't be an infinite loop.
--
Antoon Pardon
--
http://mail.python.org/mailman/listinfo/python-list
