07.07.2014 18:11, Andreas Maier wrote:
Am 07.07.2014 17:58, schrieb Xavier Morel:
On 2014-07-07, at 13:22 , Andreas Maier <andreas.r.ma...@gmx.de>
wrote:
While discussing Python issue #12067
(http://bugs.python.org/issue12067#msg222442), I learned that Python
3.4 implements '==' and '!=' on the object type such that if no
special equality test operations are implemented in derived classes,
there is a default implementation that tests for identity (as opposed
to equality of the values).
[...]
IMHO, that default implementation contradicts the definition that
'==' and '!=' test for equality of the values of an object.
[...]
To me, a sensible default implementation for == on object would be
(in Python):
if v is w:
return True;
elif type(v) != type(w):
return False
else:
raise ValueError("Equality cannot be determined in default
implementation")
Why would comparing two objects of different types return False
Because I think (but I'm not sure) that the type should play a role
for comparison of values. But maybe that does not embrace duck typing
sufficiently, and the type should be ignored by default for comparing
object values.
but comparing two objects of the same type raise an error?
That I'm sure of: Because the default implementation (after having
exhausted all possibilities of calling __eq__ and friends) has no way
to find out whether the values(!!) of the objects are equal.
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.
Cheers.
*j
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