How to Compare Custom Class Objects for Equality in Python?

Comparing Objects for Equality by Their Attributes

In Python, comparing two objects using the equality operator (==) doesn't always produce the expected result when these objects are instances of custom classes. To address this issue, we can implement the eq method to define custom equality behavior for our custom classes.

Consider the MyClass class with foo and bar attributes:

<code class="python">class MyClass:
    def __init__(self, foo, bar):
        self.foo = foo
        self.bar = bar</code>

Two instances of this class, x and y, have identical attribute values:

<code class="python">x = MyClass('foo', 'bar')
y = MyClass('foo', 'bar')</code>

However, comparing them using the equality operator results in False:

<code class="python">x == y</code>

To make Python consider these instances equal, we implement the eq method:

<code class="python">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</code>

Now, comparing x and y returns True:

<code class="python">x == y</code>

Note that implementing eq automatically makes instances of our class unhashable, preventing their storage in sets and dicts. If our class models an immutable type, we should also implement the hash method:

<code class="python">class MyClass:
    def __hash__(self):
        return hash((self.foo, self.bar))</code>

Looping through dict to compare values is discouraged as it's not truly general and can encounter uncomparable or unhashable types. Python 2 users may need to implement cmp instead of eq and consider implementing ne for inequality behavior.

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