[Python-Dev] Rich comparison confusion

[Michael Hudson]

Skip Montanaro <skip@mojam.com> writes:

> I'm a bit confused about Guido's rich comparison stuff.  In the description
> he states that __le__ and __ge__ are inverses as are __lt__ and __gt__.
> >From a boolean standpoint this just can't be so.  Guido mentions partial
> orderings, but I'm still confused.  Consider this example: Objects of type A
> implement rich comparisons.  Objects of type B don't.  If my code looks like
> 
>     a = A()
>     b = B()
>     ...
>     if b < a:
>         ...
> 
> My interpretation of the rich comparison stuff is that either
> 
>     1. Since b doesn't implement rich comparisons, the interpreter falls
>        back to old fashioned comparisons which may or may not allow the
>        comparison of B objects and A objects.
> 
>     or
> 
>     2. The sense of the inequality is switched (a > b) and the rich
>        comparison code in A's implementation is called.
> 
> That's my reading of it.  It has to be wrong.  The inverse comparison should
> be a >= b, not a > b, but the described pairing of comparison functions
> would imply otherwise.
> 
> I'm sure I'm missing something obvious or revealing some fundamental failure
> of my grade school education.  Please explain...

For a total order:

a < b if and only if b > a.
This is what the rich comparison code does.

a < b if and only if a >= b. 
This is that the rich comparison code doesn't do.

Does this make sense?

Cheers,
M.

-- 
  Presumably pronging in the wrong place zogs it.
                                        -- Aldabra Stoddart, ucam.chat