ordering with duck typing in 3.1

[Rob Williscroft]

andrew cooke wrote in
news:33019705.1873.1333801405463.JavaMail.geo-discussion-forums at ynmm9 in
gmane.comp.python.general: 

> 
> 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 __lt__(self, other):
>         return self.value < other
> 
 
> so what am i missing?
> 

The part of the docs you are relying on uses the wording "in general",
IOW, it is not saying that defining __eq__ and __lt__ will always be 
sufficient.

In this case the expression "2 < three" is calling int.__lt__, which
doesn't know how to comapre to an instance of your class so returns 
NotImplemented.

At this point if you had defined a __gt__ method the interpreter would then 
try and call that having first switched the arguments around.  But you 
didn't so a TypeError is raised.

I'm afraid I couldn't find anywhere in the docs where that behaviour is 
described, I suspect I only know it from lurking on usenet for a number of 
years.

The best description that I could find of the behaviour you are seeing is 
at:

http://docs.python.org/py3k/reference/expressions.html#not-in

There is a paragraph that contains:

"... the == and != operators always consider objects of different types to 
be unequal, while the <, >, >= and <= operators raise a TypeError when 
comparing objects of different types that do not implement these operators 
for the given pair of types. ..."

Perhapse the docs could be reworded to note that, to define a full set of 
comparisons between *different* types, you need to define a full set of 
special methods.

Some links I found along the way:

http://docs.python.org/release/3.1.3/library/constants.html?
highlight=__lt__#NotImplemented

http://code.activestate.com/recipes/576685/
http://docs.python.org/py3k/library/functools.html#functools.total_ordering

Rob.