3.4  Objects

Classes

A class is a blueprint or a template for creating objects. It defines the set of attributes (data/variables) and methods (functions/behavior) that all objects created from it will have. The keyword class is used to define a class. Class names are typically written in “CamelCase” (e.g., Car, DataModel).

class Particle:
    # This is the blueprint for a particle object
    pass

An object (or instance) is a concrete realization of a class. Creating an object from a class is called instantiation.

# Instantiation: creating two Particle objects
p1 = Particle()
p2 = Particle()

p1 and p2 are distinct objects; they have their own independent data.

Attributes

Attributes are variables that store data specific to an object. They represent the characteristics or state of the object.

The special method init (the constructor) is automatically called when a new object is created. It’s used to set up the initial state of the object, often by accepting initial values (parameters). The first parameter of any instance method, including init, must be self, which is a reference to the instance being created.

class Vector:
    # Constructor method
    def __init__(self, x_component, y_component):
        # Attributes are created and assigned to 'self'
        self.x = x_component
        self.y = y_component

# Instantiate a Vector object
v = Vector(x_component=3.0, y_component=4.0)

You access an object’s attributes using the dot notation (object.attribute_name).

print(v.x)     # Output: 3.0
v.y = 5.0      # Attributes can be modified directly
print(v.y)     # Output: 5.0

Methods

Methods are functions defined inside a class that operate on the object’s attributes. They define the behavior and capabilities of the object. Like init, all instance methods must take self as their first parameter.

class Circle:
    def __init__(self, radius):
        self.radius = radius

    # Instance method to calculate area
    def calculate_area(self):
        # Methods access attributes via 'self'
        pi = 3.14159
        return pi * self.radius ** 2

# Instantiate and call the method
c = Circle(radius=10)
area = c.calculate_area()
print(f"Area: {area}") # Output: Area: 314.159

When you call c.calculate_area(), Python automatically passes the object c as the first argument to the calculate_area method, where it is received by the self parameter. This allows the method to access and modify the specific object’s data (e.g., self.radius).