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Python

Exploring Python's Special and Dunder Methods: A Comprehensive Guide

Gideon Philip

Gideon Philip

— 7 min read
Mastering Python: A Deep Dive into Intermediate and Advanced Programming Concepts - Part I

Introduction

We've utilized a 'special method' or 'dunder method' when implementing the Python class constructor __init__(), where 'dunder' stands for 'Double Underscore.' These methods are a form of operator overloading extensively used in the Python language, with Python offering approximately 80 special methods. In this article, we'll demonstrate how special methods can be valuable in creating user-defined data objects.

To understand more about Python's special methods, we'll be exploring the following topics in this article:

  1. Introduction to Special Methods in Python
  2. Common Use Cases for Special Methods
  3. An Overview of Key Special Methods
  4. Custom Dictionary Implementation Using Special Methods
  5. Conclusion

Introduction to Special Methods in Python

What are special methods in Python?

If you are familiar with any other programming language, Python seems a little different when calling methods.

// Java implementation of length
Sting s1 = "What is this world!";
int length = s1.length();
sys.out.println(length);

# Python implementation of length
lst = [1,2,3,4,5]
print(len(lst))

In Python, we use the len(lst_obj) syntax rather than lst_obj.len(). This distinction exists because Python's list objects internally support a special method called __len__(). When you call the len function, the Python interpreter actually invokes the lst_obj.__len__() method.

Special methods in Python allow you to customize the behavior of your custom classes, making them more Pythonic and intuitive to work with. These methods can be defined in your classes to make your objects behave like built-in Python types in various contexts. Some common scenarios where you might implement special methods include:

  1. Collections
  2. Attribute access
  3. Iteration (including asynchronous iteration using async for)
  4. Operator overloading
  5. Function and method invocation
  6. String representation and formatting
  7. Asynchronous programming using await
  8. Object creation and destruction
  9. Managed contexts using the with or async with statements

Common Use Cases for Special Methods

How special methods are used in Python?

Let us start with a simple Python program to understand how you can use "special method" in your object. 

import math

class Vector:
    def __init__(self,x,y):
        self._x = x
        self._y = y
        
    def __repr__(self) -> str:
        return f'Vector({self._x},{self._y})'
        
    def __abs__(self):
        return math.hypot(self._x,self._y)
        
    def __bool__(self):
        return bool(abs(self))
        
    def __add__(self,other):
        x = self._x + other._x
        y = self._y + other._y
        return Vector(x,y)
        
    def __mul__(self,scalar):
        x = self._x * scalar
        y = self._y * scalar
        return Vector(x,y)
        
    def __sub__(self,other):
        x = self._x - other._x
        y = self._y - other._x
        return Vector(x,y)
        
    def __truediv__(self,other):
        x = self._x / other._x
        y = self._y / other._y
        return Vector(x,y)

In the program above, we've defined a custom class called Vector to represent 2D coordinates. This class includes a constructor to initialize its properties.

__init__:

  • To initialize the attributes of an object. This constructor accepts two components x and y to create instances of the vector.
def __init__(self,x,y):
        self._x = x
        self._y = y

vec1 = Vector(1,2)

__repr__:

  • We use this implemented special method to provide a clear and formal string representation of a vector object. Implementing a custom __repr__ method significantly enhances the convenience of debugging and inspecting your objects.
def __repr__(self) -> str:
    return f'Vector({self._x},{self._y})'

__abs__:

  • This method calculates the magnitude of the vector using the math.hypot function, making it easier to compute the magnitude of a vector object using the Python `abs()` function.
def __abs__(self):
    return math.hypot(self._x, self._y)

__bool__:

  • This method uses the vector's magnitude to establish whether the created object is non-zero. Returns True if the vector is non-zero or False. 
def __bool__(self):
    return bool(abs(self))

Vector class implements arithmetic special methods __add__, __sub__, __mul__, __truediv__. To empower vector objects to perform addition, subtraction, multiplication, and division. 

def __add__(self,other):
    x = self._x + other._x
    y = self._y + other._y
    return Vector(x,y)

def __mul__(self,scalar):
    x = self._x * scalar
    y = self._y * scalar
    return Vector(x,y)

def __sub__(self,other):
    x = self._x - other._x
    y = self._y - other._x
    return Vector(x,y)

def __truediv__(self,other):
    x = self._x / other._x
    y = self._y / other._y
    return Vector(x,y)

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An Overview of Key Special Methods

To enhance the Pythonic nature of your custom object. Python language incorporates over 80 distinct specialized methods, each designed for a specific purpose and catering to unique objects in distinct ways.

Refered from Fluent Python Book
Reference from Fluent Python
Reference from Fluent Python

Custom Dictionary Implementation Using Special Methods

How to Override Special Methods for Creating Custom Dictionaries?

With special methods like __getitem__, __setitem__, __delitem__, and __contains__, you can create a custom dictionary-like class in Python. Here's a simple example:

class CustomDict:
    def __init__(self):
        self.data = {}

    def __getitem__(self, key):
        if key in self.data:
            return self.data[key]
        else:
            raise KeyError(f"'{key}' not found in CustomDict")

    def __setitem__(self, key, value):
        self.data[key] = value

    def __delitem__(self, key):
        if key in self.data:
            del self.data[key]
        else:
            raise KeyError(f"'{key}' not found in CustomDict")

    def __contains__(self, key):
        return key in self.data

    def __len__(self):
        return len(self.data)

    def keys(self):
        return list(self.data.keys())

    def values(self):
        return list(self.data.values())

    def items(self):
        return list(self.data.items())

Let's elaborate on the methods used in the custom dictionary-like class CustomDict:

__getitem__(self, key):

  • This special method is invoked using square brackets to access a value by its key, like this: my_dict['name'].
  • It verifies whether the key exists in the self.data dictionary and returns the corresponding value if found.
  • In the case of a custom dictionary object, it will raise a KeyError if the key is not found.

__setitem__(self, key, value):

  • This method is called when you use square brackets to assign a key-value pair, such as my_dict['name'] = 'Alice'.
  • It either updates an existing key-value pair or adds a new one to the self.data dictionary.

__delitem__(self, key):

  • This method is invoked when you use the del statement to delete a key-value pair, like this: del my_dict['age'].
  • It removes the key-value pair if it exists in the self.data dictionary and raises a KeyError if the key is not found.

__contains__(self, key):

  • Use this special method when you want to check if a key exists in the dictionary using the in keyword, like name in my_dict.
  • It returns True if the key is in the dictionary, and False if it's not.

__len__(self):

  • This method returns the number of key-value pairs in the self.data dictionary, enabling you to use the len() function with your CustomDict objects.

keys(self):

  • This is a custom method added to the class to return a list of all keys to the dictionary.

values(self):

  • This custom method returns a list of all values in the dictionary.

items(self):

  • Another custom method that returns a list of all key-value pairs (as tuples) in the dictionary.

By implementing these special methods, along with additional custom methods, you've created a class that mimics dictionary behavior. It allows you to perform standard dictionary operations such as 'getting,' 'setting,' 'deleting,' 'checking for key existence,' and more.

We've customized this class to mimic a dictionary's behavior using special methods like __getitem__, __setitem__, __delitem__, and __contains__. You can use it to store and access key-value pairs, similar to a standard Python dictionary.

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Conclusion

In summary, Python special methods, often referred to as 'dunder methods' or 'magic methods,' are powerful tools that allow you to customize the behavior of your classes and objects. They enable you to create more intuitive and Pythonic interfaces for your custom data types, resulting in code that is more readable, maintainable, and self-explanatory. Special methods enhance the overall usability and seamless integration of your custom classes within the Python ecosystem.

About the Mastering Python Series

Welcome to the Mastering Python series, an exploration of intermediate and advanced programming concepts inspired by 'Fluent Python' by Luciano Ramalho. In this series, we delve deep into Python's nuances, covering topics such as special methods, object-oriented programming, metaclasses, decorators, and more.

Upcoming Articles

  • Part 2: Mastering Python Sequences and Generators
  • Part 3: Understanding Mapping Functions in Python

Stay tuned for more in-depth Python insights as we journey through this series. Your mastery of Python programming is just a click away!

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