Elegant ways to support equivalence ("equality") in Python classes

You need to be careful with inheritance:

>>> class Foo:
    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self.__dict__ == other.__dict__
        else:
            return False

>>> class Bar(Foo):pass

>>> b = Bar()
>>> f = Foo()
>>> f == b
True
>>> b == f
False

Check types more strictly, like this:

def __eq__(self, other):
    if type(other) is type(self):
        return self.__dict__ == other.__dict__
    return False

Besides that, your approach will work fine, that's what special methods are there for.

The way you describe is the way I've always done it. Since it's totally generic, you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality.

class CommonEqualityMixin(object):

    def __eq__(self, other):
        return (isinstance(other, self.__class__)
            and self.__dict__ == other.__dict__)

    def __ne__(self, other):
        return not self.__eq__(other)

class Foo(CommonEqualityMixin):

    def __init__(self, item):
        self.item = item

Not a direct answer but seemed relevant enough to be tacked on as it saves a bit of verbose tedium on occasion. Cut straight from the docs...

functools.total_ordering(cls)

Given a class defining one or more rich comparison ordering methods, this class decorator supplies the rest. This simplifies the effort involved in specifying all of the possible rich comparison operations:

The class must define one of __lt__(), __le__(), __gt__(), or __ge__(). In addition, the class should supply an __eq__() method.

New in version 2.7

@total_ordering
class Student:
    def __eq__(self, other):
        return ((self.lastname.lower(), self.firstname.lower()) ==
                (other.lastname.lower(), other.firstname.lower()))
    def __lt__(self, other):
        return ((self.lastname.lower(), self.firstname.lower()) <
                (other.lastname.lower(), other.firstname.lower()))

You don't have to override both __eq__ and __ne__ you can override only __cmp__ but this will make an implication on the result of ==, !==, < , > and so on.

is tests for object identity. This means a is b will be True in the case when a and b both hold the reference to the same object. In python you always hold a reference to an object in a variable not the actual object, so essentially for a is b to be true the objects in them should be located in the same memory location. How and most importantly why would you go about overriding this behaviour?

Edit: I didn't know __cmp__ was removed from python 3 so avoid it.