[Python-Dev] == on object tests identity in 3.x

[Rob Cliffe]

On 07/07/2014 22:11, Jan Kaliszewski wrote:
> [snip]
>
> IMHO, in Python context, "value" is a very vague term.  Quite often we 
> can read it as the very basic (but not the only one) notion of "what 
> makes objects being equal or not" -- and then saying that "objects are 
> compared by value" is a tautology.
>
> In other words, what object's "value" is -- is dependent on its 
> nature: e.g. the value of a list is what are the values of its 
> consecutive (indexed) items; the value of a set is based on values of 
> all its elements without notion of order or repetition; the value of a 
> number is a set of its abstract mathematical properties that determine 
> what makes objects being equal, greater, lesser, how particular 
> arithmetic operations work etc...
>
> I think, there is no universal notion of "the value of a Python 
> object".  The notion of identity seems to be most generic (every 
> object has it, event if it does not have any other property) -- and 
> that's why by default it is used to define the most basic feature of 
> object's *value*, i.e. "what makes objects being equal or not" (== and 
> !=).  Another possibility would be to raise TypeError but, as Ethan 
> Furman wrote, it would be impractical (e.g. key-type-heterogenic dicts 
> or sets would be practically impossible to work with).  On the other 
> hand, the notion of sorting order (< > <= >=) is a much more 
> specialized object property.
Quite so.

x, y = object(), object()
print 'Equal:', ' '.join(attr for attr in dir(x) if 
getattr(x,attr)==getattr(y,attr))
print 'Unequal:', ' '.join(attr for attr in dir(x) if 
getattr(x,attr)!=getattr(y,attr))

Equal: __class__ __doc__ __new__ __subclasshook__
Unequal: __delattr__ __format__ __getattribute__ __hash__ __init__ 
__reduce__ __reduce_ex__ __repr__ __setattr__ __sizeof__ __str__

Andreas, what attribute or combination of attributes do you think should 
be the "values" of x and y?
Rob Cliffe