On 2014年07月07日, at 13:22 , Andreas Maier <[email protected]> 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'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 but comparing two objects of the same type raise an error? _______________________________________________ Python-Dev mailing list [email protected] https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com