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).
The relevant code is in function do_richcompare() in Objects/object.c.
IMHO, that default implementation contradicts the definition that '==' and '!='
test for equality of the values of an object.
Python 2.x does not seem to have such a default implementation; == and != raise
an exception if attempted on objects that don't implement equality in derived
classes.
That's incorrect on two levels:
1. What Terry notes in the bug comments is that because all Python 3
types inherit from object this can be done as a default __eq__/__ne__,
in Python 2 the fallback is encoded in the comparison framework
(PyObject_Compare and friends):
http://hg.python.org/cpython/file/01ec8bb7187f/Objects/object.c#l756
2. Unless comparison methods are overloaded and throw an error it will
always return either True or False (for comparison operator), never throw.
I was incorrect for Python 2.x.
I'd like to gather comments on this issue, specifically:
-> Can someone please elaborate what the reason for that is?
-> Where is the discrepancy between the documentation of == and its default
implementation on object documented?
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.
Andy
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