Order of __eq__ Execution in Python

General Discussions
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#1

How is __eq__ Handled in Python and in What Order?

Since Python does not provide left/right versions of its comparison operators, how does it decide which function to call when comparing objects?

Here is an example code:

class A(object):
    def __eq__(self, other):
        print("A __eq__ called")
        return self.value == other

class B(object):
    def __eq__(self, other):
        print("B __eq__ called")
        return self.value == other

a = A()
a.value = 3
b = B()
b.value = 4

a == b

Output:

A __eq__ called
B __eq__ called
False

This seems to call both __eq__ functions. Can someone explain the official decision tree for how __eq__ python works and how Python decides which function to invoke in this case?

#2

In Python, when you use the == operator to compare objects, the __eq__ method gets called to determine equality. If the __eq__ method for one object returns NotImplemented, Python will automatically check the other object’s __eq__ method to try and handle the comparison.

In your example, a == b first invokes a.__eq__(b). Since a is an instance of class A, the method A.__eq__ runs first and prints "A __eq__ called". If A.__eq__ cannot handle the comparison and returns NotImplemented, Python switches to b.__eq__(a)—that is, class B’s __eq__ method.

This mechanism ensures that both classes involved in the comparison get a chance to handle the equality logic, creating a fallback system. __eq__ python effectively uses this sequence for cross-type comparisons.

#3

Python’s decision-making process for the == operator revolves around a straightforward but effective fallback system. Here’s how it works:

  1. Call the __eq__ method of the left-hand operand, A.__eq__(b) in this case.
  2. If A.__eq__ returns NotImplemented, Python calls the right-hand operand’s __eq__ method, B.__eq__(a).
  3. If neither method returns a valid result, the comparison defaults to False.

In your code, the sequence plays out as follows:

  • First, A.__eq__(b) is called, and "A __eq__ called" is printed.
  • Next, Python attempts B.__eq__(a) because both classes have their own implementations of __eq__.
  • Ultimately, both methods are executed since neither raises NotImplemented.

This logic ensures both objects get an opportunity to contribute to the equality check. By understanding this flow, you can refine the behavior of equality comparisons in your classes. This detailed fallback mechanism is a hallmark of __eq__ python implementation.

#4

The __eq__ method offers room for highly customized equality logic in Python. You can tailor its behavior based on the type of the operand or implement intricate checks to handle complex comparisons. If __eq__ in one class returns NotImplemented, Python gracefully invokes the other object’s __eq__, as shown in the previous answers.

Here’s an advanced example of customizing the equality check:

class A(object):
    def __eq__(self, other):
        if isinstance(other, B):  # Handle comparison with instances of B
            print("A __eq__ called with B")
            return self.value == other.value
        print("A __eq__ called with other type")
        return NotImplemented

With this setup:

  • If other is an instance of class B, A.__eq__ processes the equality.
  • For any other type, it returns NotImplemented, giving the other class a chance to handle the comparison.

This level of customization is especially useful for cross-class equality checks or when implementing rich comparisons. It demonstrates the flexibility of __eq__ python while allowing more granular control over equality logic.