Adding custom objects ignores __getattr__ with operators

In Python, when custom objects define the __getattr__ method, it allows you to customize attribute access and handle cases where an attribute is not directly defined on the object. However, the __getattr__ method is not invoked for special methods, including operators like +, -, *, etc., which are handled by other special methods like __add__, __sub__, __mul__, etc.

The reason why __getattr__ is not called for operators is that Python uses a specific method resolution order (MRO) to find and execute special methods like __add__. When Python encounters an operator, it directly looks for the corresponding special method without triggering __getattr__.

Here's an example to demonstrate this behavior:

pythonCopy code
class MyClass:

    def __init__(self, value):

        self.value = value



    def __getattr__(self, name):

        print(f'__getattr__ called for attribute "{name}"')

        return 42  # A default value for any undefined attribute



    def __add__(self, other):

        print('__add__ called')

        return self.value + other.value



a = MyClass(10)

b = MyClass(20)



# Attribute access, calls __getattr__

print(a.unknown_attribute)  # Output: __getattr__ called for attribute "unknown_attribute"   42



# Operator +, calls __add__

result = a + b  # Output: __add__ called



print(result)  # Output: 30

As you can see in the example, when we access the undefined attribute unknown_attribute, it triggers the __getattr__ method. However, when we use the + operator to add two MyClass objects, it directly calls the __add__ method without invoking __getattr__.

If you want to customize the behavior of operators like + for custom objects, you should implement the corresponding special methods (__add__, __sub__, etc.) in your class. These special methods are designed to handle specific operator behavior, and you cannot achieve the same effect using __getattr__.