In this code:

class A(object):
    def __init__(self):
        self.x = 'Hello'

    def method_a(self, foo):
        print self.x + ' ' + foo

... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...

a = A()               # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument

The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.

I appreciate a good cup of coffee.

Understanding __init__ and self in Python In Python's object-oriented programming (OOP), __init__ and self are fundamental concepts that facilitate the creation and management of objects. Grasping their roles is essential for writing effective and maintainable Python code. Let's explore what each of these does, why they're important, and how to use them properly. What is __init__? Definition __init__ is a special method in Python known as the initializer or constructor. It is automatically called when a new instance of a class is created. The primary purpose of __init__ is to initialize the object's attributes with values provided during instantiation. Syntax class ClassName: def __init__(self, parameters): # Initialization code self.attribute = value Example class Person: def __init__(self, name, age): self.name = name # Initializing the 'name' attribute self.age = age # Initializing the 'age' attribute # Creating an instance of Person person1 = Person("Alice", 30) print(person1.name) # Output: Alice print(person1.age) # Output: 30 Explanation: Class Definition: The Person class has an __init__ method that takes name and age as parameters. Attribute Initialization: Inside __init__, self.name and self.age are set to the values passed during object creation. Object Creation: When person1 = Person("Alice", 30) is executed, Python calls the __init__ method to initialize person1's attributes. What is self? Definition self is a conventionally named parameter in Python that refers to the instance of the class. It allows access to the attributes and methods of the class in Python. While self is not a reserved keyword, it is strongly recommended to use self to maintain readability and consistency. Role of self Accessing Instance Attributes: self is used to access or modify the attributes of the class instance. Calling Other Methods: It allows methods within the class to call other methods or access attributes. Distinguishing Between Instance and Local Variables: Helps differentiate between instance attributes and local variables within methods. Example class Circle: def __init__(self, radius): self.radius = radius # Using self to set the instance attribute def area(self): return 3.14 * (self.radius ** 2) # Accessing the instance attribute using self def resize(self, new_radius): self.radius = new_radius # Modifying the instance attribute using self # Creating an instance of Circle circle1 = Circle(5) print(circle1.area()) # Output: 78.5 circle1.resize(10) print(circle1.area()) # Output: 314.0 Explanation: Initialization: The __init__ method initializes the radius attribute using self.radius = radius. Accessing Attributes: The area method calculates the area by accessing self.radius. Modifying Attributes: The resize method changes the radius attribute using self.radius = new_radius. Why Are __init__ and self Needed? Object Initialization: __init__ allows you to set up the initial state of an object by initializing its attributes. Instance-Specific Data: self ensures that each object maintains its own state, allowing multiple instances of a class to have different attribute values. Encapsulation: Together, they promote encapsulation by bundling data (attributes) and methods that operate on the data within the class.