Difference between dunder methods (e.g., __add__) and non-dunder methods (e.g., append)

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Difference Between Dunder Methods (e.g., __add__) and Non-Dunder Methods (e.g., append)

Dunder methods (short for “double underscore” methods), also known as “magic methods,” are special methods in Python that are automatically triggered in certain situations. They are used to define how objects of a class should behave with standard Python operations (like addition, comparison, or string representation). These methods are not typically called directly by the programmer; instead, they are invoked by Python behind the scenes.

In contrast, non-dunder methods (such as append for lists) are regular methods that you call directly to perform a specific action on an object.

Let’s compare the usage and behavior of the dunder method __add__ and the non-dunder method append to better understand their differences.

Example 1: Dunder Method __add__

The __add__ method is a dunder method used to define how two objects of a class are added together using the + operator.

Example:

class Vector:
def __init__(self, x, y):
self.x = x
self.y = y

# Dunder method for addition
def __add__(self, other):
return Vector(self.x + other.x, self.y + other.y)

def __repr__(self):
return f"Vector({self.x}, {self.y})"

# Creating two Vector objects
v1 = Vector(2, 3)
v2 = Vector(4, 5)

# Using the `+` operator which calls the `__add__` method
v3 = v1 + v2 # Equivalent to v1.__add__(v2)
print(v3) # Output: Vector(6, 8)

Explanation:

• The __add__ method defines how two Vector objects are added using the + operator.
• When you write v1 + v2, Python automatically calls the __add__ method, resulting in a new Vector object with the sum of the corresponding x and y components.
• You never directly call v1.__add__(v2); instead, Python handles this when the + operator is used.

Example 2: Non-Dunder Method append

The append method is a regular method used to add an element to the end of a list. It has to be explicitly called by the programmer.

Example:

# Creating a list
numbers = [1, 2, 3]

# Using the append method to add an element to the list
numbers.append(4)

print(numbers) # Output: [1, 2, 3, 4]

Explanation:

• The append method is not a dunder method. You need to explicitly call it using numbers.append(4) to add an element to the numbers list.
• The append method performs a specific action (adding an element to the end of a list) and does not involve operator overloading like dunder methods.

Key Differences Between Dunder and Non-Dunder Methods:

1. Invocation:

– Dunder Method (__add__): Triggered automatically by Python in response to an operator (e.g., + invokes __add__). You generally do not call dunder methods directly.
– Non-Dunder Method (append): Must be explicitly called by the programmer (e.g., numbers.append(4)).

1. Purpose:

– Dunder Method: Often used to define how objects interact with built-in operators or Python-specific actions (e.g., addition, comparison, string representation).
– Non-Dunder Method: Performs specific tasks defined by the class (e.g., append adds an item to a list).

1. Customization:

– Dunder Method: Enables operator overloading and allows you to define custom behavior for operators in your classes (e.g., +, -, ==, >, etc.).
– Non-Dunder Method: Provides general functionality for manipulating the object’s internal data (e.g., adding, removing, modifying elements).

Side-by-Side Example:

Let’s consider how __add__ and append would be used in a custom class.

Custom Class with Both __add__ and append Methods:

class Bag:
def __init__(self):
self.items = []

# Non-dunder method to append items
def append(self, item):
self.items.append(item)

# Dunder method to define '+' behavior
def __add__(self, other_bag):
combined_bag = Bag()
combined_bag.items = self.items + other_bag.items
return combined_bag

def __repr__(self):
return f"Bag({self.items})"

# Creating two Bag objects
bag1 = Bag()
bag2 = Bag()

# Using the non-dunder append method
bag1.append('apple')
bag2.append('banana')

print(bag1) # Output: Bag(['apple'])
print(bag2) # Output: Bag(['banana'])

# Using the '+' operator to combine two bags (triggers __add__ method)
combined_bag = bag1 + bag2
print(combined_bag) # Output: Bag(['apple', 'banana'])

Explanation:

• The append method is explicitly called by the programmer to add items to the bag.
• The + operator is used to combine two Bag objects, and this triggers the __add__ method automatically. This method combines the items of both bags into a new Bag object.

Summary:

• Dunder Methods (__add__): Automatically triggered by built-in operators like +, -, *, etc. They enable you to define how objects of your custom class should behave with standard Python operations.
• Non-Dunder Methods (append): Explicitly called by the programmer to perform a specific task or action on an object. They provide functionality directly related to the object’s internal data.

While dunder methods are powerful for customizing operator behavior, non-dunder methods provide a more straightforward way to manipulate and work with objects.

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