[Python-Dev] == on object tests identity in 3.x
python-dev at masklinn.net
Mon Jul 7 17:58:39 CEST 2014
On 2014-07-07, at 13:22 , Andreas Maier <andreas.r.maier at 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):
2. Unless comparison methods are overloaded and throw an error it will
always return either True or False (for comparison operator), never throw.
> 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
> raise ValueError("Equality cannot be determined in default implementation")
Why would comparing two objects of different types return False but
comparing two objects of the same type raise an error?
More information about the Python-Dev