Python Equality

operand_a == operand_b

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Internal Mechanics

1. Subclass Priority Rule: If operand_b is an instance of a strict subclass of operand_a’s type, Python reverses the standard dispatch order. It will call type(operand_b).__eq__(operand_b, operand_a) first. This ensures that subclasses can properly override and dictate the equality behavior of their parent classes.
2. Primary Dispatch: If the subclass rule does not apply, Python attempts to call the __eq__ method of the left operand: type(operand_a).__eq__(operand_a, operand_b).
3. Fallback Dispatch: If the first method called returns the built-in NotImplemented constant (indicating it does not know how to compare itself to the other type), Python falls back to the other operand’s __eq__ method.
4. Identity Fallback: If both operands return NotImplemented, Python falls back to comparing object identity, effectively evaluating operand_a is operand_b (comparing their memory addresses).

# The conceptual equivalent of the == operator's internal logic
def evaluate_equality(a, b):
    type_a = type(a)
    type_b = type(b)
    
    # 1. Subclass Priority Rule
    if type_a is not type_b and issubclass(type_b, type_a):
        result = type_b.__eq__(b, a)
        if result is not NotImplemented:
            return result
        result = type_a.__eq__(a, b)
        
    # 2. Standard Dispatch
    else:
        result = type_a.__eq__(a, b)
        if result is NotImplemented:
            # 3. Fallback Dispatch
            result = type_b.__eq__(b, a)
            
    # 4. Identity Fallback
    if result is NotImplemented:
        return a is b
        
    return result

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Type Handling and Coercion

• Numeric Types: Python’s numeric types (int, float, complex, decimal, fractions) implement cross-type equality. The == operator will evaluate 1 == 1.0 as True because their mathematical values are equivalent, despite differing memory representations.
• Collections: For built-in container types (like list, tuple, dict, and set), the == operator performs a deep, recursive equality check. It iterates through the collections, invoking the == operator on each corresponding pair of elements. Both the structure and the values must match.
• Custom Objects: By default, user-defined classes inherit their __eq__ method from the base object class. The default implementation strictly compares memory addresses. To achieve value equality in custom objects, the developer must explicitly override the __eq__ method to define the specific attributes that constitute equality.

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Hashing Implications