What does a special attribute starting with double underscores mean in a programming language?

In Python, attributes or methods that start and end with double underscores (e.g., __init__, __str__) are called "magic" or "special" methods. They have a special significance for the interpreter and allow overriding the behavior of objects, such as initialization, string representation, arithmetic operations, etc. These methods are not intended to be called directly by the user but are used by built-in functions and operators.

Example:

class MyClass:
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return f"MyClass with value = {self.value}"

obj = MyClass(10)
print(obj)  # Calls __str__ automatically