Constraints on __sub__, __eq__, etc.

[Robert Kern]

On 2010-02-18 10:19 AM, Andrey Fedorov wrote:
> It seems intuitive to me that the magic methods for overriding the +, -,
> <, ==, >, etc. operators should have no sideffects on their operands.
> Also, that == should be commutative and transitive, that > and < should
> be transitive, and anti-commutative.
>
> Is this intuition written up in a PEP, or assumed to follow from the
> mathematical meanings?

Some of it is covered in the reference manual. E.g.

   http://docs.python.org/reference/datamodel.html#object.__lt__

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco