ordering with duck typing in 3.1

[John O'Hagan]

On Sat, 7 Apr 2012 05:23:25 -0700 (PDT)
andrew cooke <andrew at acooke.org> wrote:

> 
> hi,
> 
> please, what am i doing wrong here?  the docs say
> http://docs.python.org/release/3.1.3/library/stdtypes.html#comparisons "in
> general, __lt__() and __eq__() are sufficient, if you want the conventional
> meanings of the comparison operators" but i am seeing
> 
> >       assert 2 < three
> E       TypeError: unorderable types: int() < IntVar()
> 
> with this test:
> 
> 
> class IntVar(object):
> 
>     def __init__(self, value=None):
>         if value is not None: value = int(value)
>         self.value = value
> 
>     def setter(self):
>         def wrapper(stream_in, thunk):
>             self.value = thunk()
>             return self.value
>         return wrapper
> 
>     def __int__(self):
>         return self.value
> 
>     def __lt__(self, other):
>         return self.value < other
> 
>     def __eq__(self, other):
>         return self.value == other
> 
>     def __hash__(self):
>         return hash(self.value)
> 
> 
> class DynamicTest(TestCase):
> 
>     def test_lt(self):
>         three = IntVar(3)
>         assert three < 4
>         assert 2 < three
>         assert 3 == three
> 
> so what am i missing?
> 

I think that quote from the docs is just to point out that you only need those
two (== and <) to derive any of the other comparisons; but not to imply that a
class that only defines those two will automatically possess the others.
However, you can do that, with functools.total_ordering.

Regards,

John