Keeping Your Python Classes Equal: A Comprehensive Guide to Equivalence Methods

In Python, the eq and ne special methods provide a convenient way to define equivalence for custom classes. While the basic approach of comparing __dict__s is a viable option, it may face challenges with subclasses and interoperability with other types.

A More Robust Equivalence Handling

To address these limitations, consider a more comprehensive implementation:

class Number:

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

    def __eq__(self, other):
        if isinstance(other, Number):
            return self.number == other.number
        return NotImplemented

    def __ne__(self, other):
        x = self.__eq__(other)
        if x is not NotImplemented:
            return not x
        return NotImplemented

    def __hash__(self):
        return hash(tuple(sorted(self.__dict__.items())))

class SubNumber(Number):
    pass

This version includes:

• Subclass Handling: It ensures that equivalence is checked properly between the Number class and its subclasses.
• Non-commutative Correction: It ensures that equality comparisons are commutative, regardless of the operand order.
• Hash Overriding: By defining a custom hash method, it ensures that objects with the same value have the same hash value, which is crucial for set and dictionary operations.

Validation and Testing

To verify the robustness of this approach, here's a set of assertions:

n1 = Number(1)
n2 = Number(1)
n3 = SubNumber(1)
n4 = SubNumber(4)

assert n1 == n2
assert n2 == n1
assert not n1 != n2
assert not n2 != n1

assert n1 == n3
assert n3 == n1
assert not n1 != n3
assert not n3 != n1

assert not n1 == n4
assert not n4 == n1
assert n1 != n4
assert n4 != n1

assert len(set([n1, n2, n3])) == 1
assert len(set([n1, n2, n3, n4])) == 2

These assertions demonstrate the correct behavior of the equivalence methods and the consistency of hash values.

By embracing this more comprehensive approach, you can create Python classes with robust equivalence handling, ensuring reliable comparisons and accurate set and dictionary operations.