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Dunder Methods

In this tutorial, we will explore the concept of dunder methods, also known as magic methods, and their application in Python classes. Dunder methods, short for "double underscore" methods, are special methods with double leading and trailing underscores (e.g., __init__). These methods are integral to defining the behavior of objects in Python. While we've already encountered the __init__ method for initializing instances, we will delve deeper into other dunder methods that enhance the functionality and interactivity of our classes.

Introduction to Dunder Methods​

Dunder methods enable customization of fundamental object behaviors, such as initialization, representation, arithmetic operations, and more. It's crucial to note that these methods are not called directly by their names. Instead, they are invoked implicitly through specific syntax or functions in Python.

Example Class: Book​

To demonstrate the use of dunder methods, we'll define a Book class. This class will include several dunder methods to illustrate their functionalities.

Step 1: Defining the __init__ Method​

The __init__ method initializes class instances. Here, we'll define a Book class with attributes for the title and number of pages.

class Book:
    def __init__(self, title: str, pages: int) -> None:
        self.title = title
        self.pages = pages

Step 2: Implementing the __len__ Method​

The __len__ method allows the use of the len() function on instances of the class. In our Book class, it will return the number of pages.

    def __len__(self) -> int:
        return self.pages

Step 3: Testing the __len__ Method​

We can test our __len__ method by creating instances of the Book class and using the len() function.

if __name__ == "__main__":
    pi_daily = Book("Pi Daily", 100)
    harry_potter = Book("Harry Potter", 340)

    print(len(pi_daily))  # Output: 100
    print(len(harry_potter))  # Output: 340

Step 4: Implementing the __add__ Method​

The __add__ method allows us to use the + operator to add instances of the class. In our Book class, it will combine the titles and sum the pages of two books.

    def __add__(self, other: 'Book') -> 'Book':
        combined_title = f"{self.title} and {other.title}"
        combined_pages = self.pages + other.pages
        return Book(combined_title, combined_pages)

Step 5: Testing the __add__ Method​

We can test our __add__ method by creating instances of the Book class and using the + operator.

if __name__ == "__main__":
    combined_books = pi_daily + harry_potter
    print(combined_books.title)  # Output: Pi Daily and Harry Potter
    print(combined_books.pages)  # Output: 440

Full Implementation​

Below is the full implementation of our Book class with the __init__, __len__, and __add__ methods:

class Book:
    def __init__(self, title: str, pages: int) -> None:
        self.title = title
        self.pages = pages

    def __len__(self) -> int:
        return self.pages

    def __add__(self, other: 'Book') -> 'Book':
        combined_title = f"{self.title} and {other.title}"
        combined_pages = self.pages + other.pages
        return Book(combined_title, combined_pages)

if __name__ == "__main__":
    pi_daily = Book("Pi Daily", 100)
    harry_potter = Book("Harry Potter", 340)

    print(len(pi_daily))  # Output: 100
    print(len(harry_potter))  # Output: 340

    combined_books = pi_daily + harry_potter
    print(combined_books.title)  # Output: Pi Daily and Harry Potter
    print(combined_books.pages)  # Output: 440

Conclusion​

Dunder methods are powerful tools in Python that allow developers to customize and extend the behavior of classes. By implementing methods such as __init__, __len__, and __add__, we can create more intuitive and functional objects. Understanding and utilizing these methods effectively can greatly enhance the design and usability of your classes.