How to Define Custom Object Equality in Python: Should You Implement `__eq__`?

Enhancing Object Equality by Attribute Comparison

In Python, comparing two objects for equality (using the == operator) typically defaults to a comparison of their object references. However, this may not be desirable when objects of the same class have identical attributes but distinct object references.

Customizing Object Equality

To modify the behavior of object equality, we can implement the eq method within the class definition. By defining a custom eq method, we specify the exact criteria for determining equality among instances of that class.

Implementation for MyClass

In the provided example, we have the MyClass class with member variables foo and bar. We can implement the eq method as follows:

class MyClass:
    def __init__(self, foo, bar):
        self.foo = foo
        self.bar = bar

    def __eq__(self, other):
        if not isinstance(other, MyClass):
            return NotImplemented
        
        return self.foo == other.foo and self.bar == other.bar

With this implementation, we are checking that both objects are instances of MyClass and then comparing their foo and bar values. If these attributes match, the objects are considered equal, and the eq method returns True.

Implications and Considerations

Implementing eq makes instances of your class unhashable, meaning they cannot be stored in sets or dictionaries. If your class represents a mutable type, you may want to consider leaving your instances unhashable.

For immutable data models, it is recommended to also implement the hash method to enable instances to behave correctly in dictionaries and sets.

Conclusion

By customizing the eq method, we gain control over how objects of a specific class are compared for equality. This allows us to define equality based on the relevant attributes of our objects, ensuring that they are considered equal when their attributes match, regardless of their object references.

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