comparison with None

[Gabriel Genellina]

En Fri, 20 Apr 2007 23:48:02 -0300, Alex Martelli <aleax at mac.com> escribió:

> Gabriel Genellina <gagsl-py2 at yahoo.com.ar> wrote:
>
>> Reference Manual, 5.9 Comparisons
>>
>> "The objects need not have the same type. If both are numbers, they are
>> converted to a common type. Otherwise, objects of different types always
>> compare unequal, and are ordered consistently but arbitrarily."
>>
>> (Apart from the latter not menctioning string types too, looks good to  
>> me).
>
> Right.  However, it might be worth underscoring that this applies to
> _built-in_ types, and user-defined types are free to implement different
> comparison semantics, although that should be done with care and good
> taste; for example, one might have a collection type defining __eq__ to
> mean "the same set of items as the other [iterable] operand in any
> order", though that might cause weird behavior such as a==b but b!=a
> (unfortunately there's no __req__, and __coerce__ is not involved in
> comparisons either).

BTW, I think that the relationship between __eq__ and __hash__ (a==b =>  
hash(a)==hash(b), when hashable) should be more clearly stated too. I  
almost got insane some time ago because of a faulty Rational numbers  
implementation where hash(1/2)!=hash(2/4) (so dictionaries didn't work at  
all).

-- 
Gabriel Genellina